In a food processor or blender, pulse bread, butter, 1/4 teaspoon salt, and 1/8 teaspoon pepper until coarse crumbs form (about 10 pulses). Transfer to a large bowl. Add onions and toss to combine.
To make the casserole:
Adjust an oven rack to middle position and preheat oven to 425 degrees. In a Dutch oven or stock pot, bring 4 quarts water and 2 tablespoons salt to boil. Fill a large bowl with ice water.
Add green beans and cook until bright green and tender-crisp, about 6 minutes. Drain in colander and immediately transfer to ice water. Spread on paper towels to dry.
To the same pot, melt the butter over medium-high heat until foaming. 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 and reduced to 3 1/2 cups, about 10 to 15 minutes. Season to taste with salt and pepper.
Add the green beans to the pot with the mushroom sauce and toss until evenly coated. Pour into a 3-quart or 9-inch by 13-inch baking dish. Sprinkle with topping.
Bake until the top is golden brown and the sauce is bubbling around the edges, about 15 minutes. Serve immediately.
Video
Notes
Butter: Salted or unsalted butter? Use what you have on hand. To soften butter in the microwave, cut each stick of butter in half, unwrap, and place on a microwave-safe plate. Then cook the butter at 10% power for 1 minute. Gently press on the butter with your finger, and if it still feels too firm, cook for another 40 seconds at 10% power.
Green beans: To substitute frozen green beans, microwave 2 (12-ounce) bags of frozen cut green beans 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).
Chicken broth: Do not use low-sodium chicken broth or the sauce may be bland. Homemade chicken broth is fine or you could mix up a batch with Better than Bouillon chicken base (Culinary Hill may earn money if you buy through this link). For a vegetarian version, substitute vegetable broth.
Mushrooms: Use fresh mushrooms to make a quick cream of mushroom soup from scratch for a flavorful addition to this Thanksgiving staple.
Yield: This recipe makes enough for 10 as a side dish (or more depending on how many sides you have at Thanksgiving).
Storage: Store leftovers covered in the refrigerator for up to 4 days.