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 to 7 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 about 10 cups of casserole, enough for 10 1-cup sized, side dish (you'll be able to feed more than 10 people depending on how many sides you have at Thanksgiving).
Storage: Store leftovers covered in the refrigerator for up to 4 days.
Make ahead: You can make the mushroom sauce up to 2 days before. Cool and refrigerate. Since the sauce will be going in cold, increase the crockpot time to 3 hours on HIGH or 5 hours on LOW.