If making ahead. Do not bake. Cover with plastic wrap, and refrigerate for up to 24 hours. To serve, remove plastic wrap and heat the casserole in a 350° oven for 30 minutes, then add the topping as directed.

To serve, heat casserole in a 350° oven for 10 minutes uncovered, then add the topping. Bake until top is golden brown and sauce is bubbling around edges, about 5 minutes.

For the Topping: Pulse bread, butter, salt, and pepper in food processor until mixture resembles coarse crumbs, about 10 1-second pulses. Transfer to a large bowl and toss with onions and set aside. Store in large zip-lock bag.

For the Beans and Sauce:

Fill large bowl with cold water. Bring 4 quarts of water to a boil in a large Dutch oven. Add 1 tsp salt and beans. Cook beans until bright green and crisp-tender, about 8 minutes. Drain beans in colander and plunge immediately into cold 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. Add garlic, 3/4 tsp salt and 1/8 tsp pepper. Sauté 1 minute. Add flour and cook for 1 minute, stirring constantly with wire whisk. Stir in broth and bring to a 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.

Add green beans to sauce and stir until evenly coated. Arrange in an even layer in a 13x9 inch baking dish. Cover with plastic wrap. Refrigerate up to 24 hours.