For the topping: Pulse bread, butter, salt, and pepper in food processor until mixture resembles coarse crumbs, about 10 one-second pulses. Transfer to large bowl and toss with onions; set aside.
Adjust oven rack to middle position and heat oven to 425 degrees.
Fill large bowl with ice water. Bring 4 qts. of water to boil in large Dutch oven. Add two tbls. salt and the green 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.
Add butter to now-empty Dutch oven and melt over medium-high heat until foaming subsides. Add mushrooms, garlic, 3/4 teaspoons salt, and 1/8 teaspoons 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.
Add green beans to sauce and stir until evenly coated. Arrange in even layer in 3 quart (or 13 x 9") baking dish. Sprinkle with topping and bake until top is golden brown ans sauce is bubbling around edges, about 15 minutes. Serve immediately.
To prepare ahead: Store breadcrumb topping in an airtight container in the refrigerator and combine with the onion 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 plastic wrap and heat 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" square) baking dish. Reduce cooking time of the sauce in Step 3 to about 6 minutes and the baking time in Step 4 to 10 minutes.