The components of the casserole can be prepared ahead of time. Store the bread-crumb topping in an airtight container in the refrigerator and combine with the onions just before cooking. Combine the beans and cooled sauce in a baking dish, cover with plastic wrap, and refrigerate for up to 24 hours. To serve, remove the plastic wrap and heat the casserole in a 425-degree oven for 10 minutes, then add the topping and bake as directed. This recipe can be halved and baked in a 2-quart (or 8-inch-square) baking dish. If making a half batch, reduce the cooking time of the sauce in step 3 to about 6 minutes (1 3/4 cups) and the baking time in step 4 to 10 minutes.
1. FOR THE TOPPING: Pulse bread, butter, salt, and pepper in food processor until mixture resembles coarse crumbs, about ten 1-second pulses. Transfer to large bowl and toss with onions; set aside.
2. FOR THE BEANS AND SAUCE: Adjust oven rack to middle position and heat oven to 425 degrees. Fill large bowl with ice water. Bring 4 quarts water to boil in large Dutch oven. Add 2 tablespoons salt and beans. Cook beans until bright green and crisp-tender, about 6 minutes. Drain beans in colander and plunge immediately into ice water to stop cooking. Spread beans on paper-towel-lined baking sheet to drain.
3. Add butter to now-empty Dutch oven and melt over medium-high heat until foaming subsides. Add mushrooms, garlic, 3/4 teaspoon salt, and 1/8 teaspoon pepper; cook until mushrooms release moisture and liquid evaporates, about 6 minutes. Add flour and cook for 1 minute, stirring constantly. Stir in broth and bring to simmer, stirring constantly. Add cream, reduce heat to medium, and simmer until sauce is thickened and reduced to 3 1/2 cups, about 12 minutes. Season with salt and pepper to taste.
4. Add green beans to sauce and stir until evenly coated. Arrange in even layer in 3-quart (or 13 by 9-inch) baking dish. Sprinkle with topping and bake until top is golden brown and sauce is bubbling around edges, about 15 minutes. Serve immediately.