In a large skillet or Dutch oven, melt 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). Drain well (I like to use a salad spinner to spin them dry).
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.
Frozen green beans: To substitute fresh beans, trim and chop 2 pounds of green beans. Blanch the beans in 4 quarts of boiling salted water for about 5 minutes, then drain well and plunge immediately in to an ice bath to stop the cooking.
Fried onions: To substitute bread crumbs for the canned fried onions, pulse 4 slices hearty white bread (torn into pieces) in a food processor or blender. You will have about 3 cups of crumbs. Stir in 3 tablespoons melted butter or olive oil and fry in a large skillet over medium-high heat until browned. Sprinkle over the casserole just before serving.
Yield: This recipe makes enough for 10 as a side dish (or more depending on how many sides you have at Thanksgiving).
Make the mushroom sauce ahead: Cool and refrigerate overnight. Increase the crockpot time to 3 hours on HIGH or 5 hours on LOW.
Blanch fresh green beans ahead: If using fresh beans instead of frozen, they may be blanched the night before, cooled, and stirred in the cooled mushroom sauce and refrigerated overnight. Increase the crockpot time to 3 to 4 hours on HIGH or 5 to 6 hours on LOW .
Storage: Store leftovers in the refrigerator for up to 4 days.
Oven instructions: Follow the recipe as written through Step 5, but then instead of transferring the green bean mixture to a slow cooker, spread it in the bottom of a 3-quart or a 9x13 baking dish. Sprinkle with fried onions and bake at 425 degrees until the top is golden brown and the sauce is bubbling around the edges, about 15 minutes.
Small army: 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, 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, and 3 cups canned fried onions (or more if desired).