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 a 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°F. Fill a large bowl with ice water. Bring 4 quarts of water to a boil in a large pot or 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 pot and melt over medium-high heat until foaming subsides. Add mushrooms, garlic, ¾ 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 3½ cups, about 12 minutes. Season with salt and pepper to taste.

4. Add green beans to sauce and stir until evenly coated. Arrange in an even layer in 3-quart (or 9x13-inch) baking dish. Sprinkle with topping and bake until top is golden brown and sauce is bubbling around edges, about 15 minutes. Serve immediately.

*Note 1: You can prep this 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°F oven for 10 minutes, then add the topping and bake as directed.

*Note 2: 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 (1¾ cups) and the baking time in Step 4 to 10 minutes.

*Note 3: If you want to go 100% homemade and eliminate the canned fried onions, you can make your own caramelized onions. Thinly slice 2 sweet onions and saute in 2 tablespoons of unsalted butter over medium-high heat for about 10 minutes. Reduce heat to medium-low and sprinkle with salt and cook, stirring occasionally, for about 30 minutes, or