Slow Cooker Green Bean Casserole, a classic Thanksgiving side dish, goes from default to downright delicious in this updated recipe. It’s made from scratch, can be made ahead, and takes up zero real estate in the oven.
Making Green Bean Casserole for a crowd? Click and slide the number next to “servings” on the recipe card below to adjust the ingredients to match how many you’re feeding—the recipe does the math for you, it’s that easy.
Change #1: the green beans:
In my experiments, frozen cut green beans worked the best.
Why? Because they’re already prepped and cut, then flash frozen for great taste.
Because canned green beans would turn to goopy mush in the slow cooker. (Plus they’re loaded with sodium.)
Fresh green beans would be great, but no way, no how would my sister Monica, the cook in charge of Green Bean Casserole every year, go for that. Her entire life would be spent trimming enough beans for everybody, only to start again for the next year on Black Friday. And is she wouldn’t I knew there wouldn’t be many who would.
Frozen green beans are the best. All you have to do is pop them in the microwave before adding them to the crock pot. They hold their shape, they’re easy to find, and they’re super convenient.
Change #2: the mushroom sauce:
We should thank Campbell’s Soup Company for the Green Bean Casserole, specifically Dorcas Reilly, a woman who worked at the test kitchen at Campbell’s and who invented the Green Bean Casserole recipe.
Back in 1955, post-war America was hungry for unfussy recipes that could be made in a hurry. As a result, casseroles made with canned soup became very popular.
But we can do better than that. We can make our own cream of mushroom soup. It’s as easy as sautéing some good mushrooms with garlic and butter, then adding a little flour to thicken things, then broth, then cream.
You can make it the night before.
Change #3: crispy onions:
I’m kidding here. The onions are stying put, because they’re a defining feature of this recipe. The only change is when they’re added to the recipe.
Oven-baked green bean casserole keeps the onions crispy while in the oven. But with a slow cooker, there’s too much moisture to keep anything crispy.
So, all you have to do is add them to the casserole right before serving. Brilliant!
Ready to cook? Let’s do it.
Green Bean Casserole ingredients:
Here’s what goes in Green Bean casserole:
- Frozen green beans. Buy the “frozen cut” green beans, not the whole ones. For one recipe which makes 8 to 10 servings, you need 2 12-ounce packages of frozen green beans, or 2 10-ounce packages of green beans (if you want to substitute fresh greens, use 2 pounds and be sure to blanch them first).
- Mushrooms. Button is fine, fancy is fancier.
- Chicken broth. Low sodium is best.
- Heavy cream. Some cooks love using half and half.
- Fried onions. These usually come in a can, stored dry, like nuts.
Some other fun ingredients you might add, depending on what your family is used to:
- Water chestnuts.
- Browned ground beef.
- Cooked chopped bacon.
- Sour cream.
- Cream cheese.
How to make the homemade mushroom sauce for Green Bean Casserole:
(This is a photo walk-through, but looking for exact amounts? See the recipe card below, as well as some good formulas for doubling and tripling the recipe.)
You can also check out my more detailed post here: How to Make Cream of Mushroom Soup.
- In a large, deep skillet, melt the butter over medium-high heat until it stops foaming. Add the sliced mushrooms, garlic, and some salt and pepper.
- Cook the mushrooms, stirring occasionally, until they release their liquid. Then stir in the flour and cook about a minute longer.
- Next, whisk in the broth and scrape up any brown bits on the bottom of the pan.
- Bring the mixture to a boil, stirring constantly. Then reduce the heat to medium.
- Whisk in the cream and simmer until the sauce gets nice and thick—about 5 minutes or so.
- Finally season to taste with salt and pepper. Remove from heat and use immediately, or let cool and refrigerate overnight.
- (The mushroom sauce may be prepared in advance, cooled, and refrigerated overnight. Make sure you increase the crock pot time to 3 hours on HIGH or 5 hours on LOW.)
How to make Slow Cooker Green Bean Casserole:
- Once you have the homemade mushroom soup sauce made, the rest is a breeze.
- Cook the green beans in the microwave according to the instructions on the package (or on HIGH power for 4 to 5 minutes)
- They’ll probably be wet, so drain them well using a colander or a salad spinner. Discard any liquid.
- Next, add the green beans to the crock pot and cover with the mushroom sauce. Gently toss until combined.
- Cover and cook on HIGH for 2 hours or LOW for 4 hours. Before serving, check the temperature; the center of the casserole should be 165 degrees.
- Finally, sprinkle on the canned fried onions and serve immediately. Don’t be shy with those onions!
Making Slow Cooker Green Bean Casserole in advance:
This recipe is fabulous because you can make the cream of mushroom sauce the day before. Store everything in the refrigerator until you are ready to cook.
Cold ingredients need a little extra time, however. Increase the crock pot time to 3 hours on HIGH or 5 hours on LOW.
How to make Slow Cooker Green Bean Casserole with fresh green beans:
You know what the old adage is: many hands make light the work? Well, grab some hands and put them to work trimming and chopping all the beans you need to make enough for your family. More power to you!
- To substitute fresh beans, use 2 pounds fresh green beans and start by trimming them and cutting them in half.
- Then, you need to blanch them. Bring 4 quarts of salted water to a boil.
- Meanwhile, fill a large bowl with ice and water and keep it nearby.
- Cook the beans about 5 minutes, until they’re bright green and crisp-tender.
- Drain the beans and immediately plunge into the ice water.
- Make sure they’re drained well before adding them to the sauce in Step 5. (You may have to pat them dry.)
Make ahead tip: If using fresh beans, they may be blanched the night before, cooled, and stirred into the cooled mushroom sauce and refrigerated overnight. In that case, increase the crock pot time to 3 hours on HIGH or 5 hours on LOW.
Substitutions for fried onions:
I’ve learned from my readers that canned fried onions aren’t available absolutely everywhere. Sometimes Asian grocery stores and Vietnamese markets sell packages of fried shallots—that may be a place to check.
But using crunchy, buttery bread crumbs is equally delicious. You can make them ahead of time and store them in the refrigerator, then return them to the oven to crisp them up for a few minutes before you add them to the crock pot.
Here’s what I would suggest:
- Preheat the oven to 350 degrees F/175 degrees C.
- Pulse 4 slices white sandwich bread (torn into quarters), 2 Tablespoons softened unsalted butter, 1/4 teaspoon salt, and 1/8 teaspoon pepper in a food processor until it resembles coarse crumbs (about 10 pulses).
- Spread the bread crumbs in an even layer on a rimmed baking sheet. Bake until lightly golden brown, about 10 to 15 minutes.
- Add them to the slow cooker right at the end, in place of the canned fried onions.
Slow Cooker Green Bean Casserole Recipe
- 3 tablespoons butter
- 1 pound white button mushrooms wiped clean, sliced or broken into pieces
- 3 cloves garlic minced
- salt and freshly ground black pepper
- 3 tablespoons all-purpose flour
- 1 1/2 cups chicken broth
- 1 1/2 cups heavy cream
- 2 (12 ounce) packages frozen cut green beans (see notes)
- 3 cups canned fried onions
- n a large, deep skillet, saucepan, or Dutch oven, melt the butter over medium-high heat until foaming subsides. Add mushrooms, garlic, 3/4 teaspoon salt, and 1/8 teaspoon pepper.
- Cook until the mushrooms have released most of their liquid, about 5 minutes. Stir in flour and cook for one minute. Whisk in broth, scraping up any brown bits from the bottom of the pan. Bring to a boil, whisking constantly.
- Reduce heat to medium. Whisk in cream and simmer until the sauce has thickened, about 5 minutes. Season to taste with salt and pepper.
- Meanwhile, cook the beans in the microwave according to the package instructions (or microwave on HIGH power for 4 to 5 minutes; let stand 1 minute before removing from the microwave). Transfer to a colander or salad spinner and drain well. If using a salad spinner, spin out any liquid and drain.
- Add the green beans to the pot with the mushroom sauce and toss until evenly coated. Transfer the beans and sauce to a crock pot.
- Cook on HIGH for 2 hours or LOW for 4 hours or until a thermometer inserted in to the middle of the casserole reaches 165 degrees. Top with canned fried onions immediately before serving.
- For the most part, I found frozen green beans in 12-ounce packages. If you find 10-ounce packages, I would still stick to 2 packages. I have tried the recipe with 3 (10 ounce) packages and there was not enough sauce for that many beans.
- Be sure to use frozen cut green beans, not “French” green beans (the long, stringy ones) or whole, uncut beans (you’d just have extra work cutting them).
- To substitute fresh beans, use 2 pounds of fresh green beans and start by trimming them and cutting them in half. Then, bring 4 quarts water and 1 Tablespoon salt to a boil. Meanwhile, fill a large bowl with ice and water. Cook the beans about 5 minutes, until bright green and crisp-tender. Drain the beans and immediately plunge into the ice water. Drain well before adding them to the sauce in Step 5.
- To triple the recipe for 30 to 40 people, increase the ingredients as follows: 1/2 cup (1 stick) butter 3 pounds white button mushrooms, wiped clean and sliced or broken into pieces 1 bulb garlic (9 to 10 cloves) 3 teaspoons salt and 1/2 teaspoon pepper 1 cup flour 4 1/2 cups chicken broth 4 1/2 cups heavy cream 6 (12 ounce) packages of frozen cut green beans 3 cups canned fried onions (or more if desired)
- 3x recipe instruction changes: Cook the mushrooms for about 10 minutes in Step 2 and simmer the sauce for 10 minutes in Step 3. Cook on HIGH for 2 hours or LOW for 4 hours (or until a thermometer reaches at least 165 degrees in the middle of the casserole). Add more canned onions throughout serving time if the top layer is depleted, if desired.
- The mushroom sauce may be prepared in advance, cooled, and refrigerated overnight. Increase the crock pot time to 3 hours on HIGH or 5 hours on LOW.
- If using fresh beans, they may be blanched the night before, cooled, and stirred into the cooled mushroom sauce and refrigerated overnight. Increase the crock pot time to 3 hours on HIGH or 5 hours on LOW.
- Adapted from Cook’s Illustrated Magazine, November & December 2006 issue.