For the topping:
4 slices white sandwich bread, each slice torn into quarters
2 tablespoons unsalted butter, softened
¼ teaspoon table salt
⅛ teaspoon ground black pepper
3 cups canned fried onions (about 6 ounces)
For the beans:
2 pounds green beans, ends trimmed, halved
3 tablespoons unsalted butter
1 pound white button mushrooms, stems trimmed, wiped clean, and broken into ½-inch pieces
3 medium cloves garlic, minced (about 1 tablespoon)
Salt and freshly ground black pepper
3 tablespoons unbleached all-purpose flour
1½ cups low-sodium chicken broth
1½ cups heavy cream
In the bowl of a food processor, pulse together the bread, butter, salt and pepper until coarse crumbs form, about ten 1-second pulses. Transfer to a large bowl and toss with the onions; set aside.
Heat oven to 425º. Prepare an ice bath by filling a large bowl with ice water and place it near the stove.
In a large Dutch oven, bring 4 quarts of water to a boil. Add 2 tablespoons of salt and beans. Cook for about 6 minutes, until the beans are bright green and crisp-tender. Drain the beans and dunk them into the water to stop cooking. Remove from the ice bath and place onto a paper towel lined baking sheet to drain completely.
Add butter to Dutch oven and melt over medium high heat. Once foaming has subsided, add mushrooms, garlic, ¾ teaspoon salt and ⅛ teaspoon of pepper. Cook for about 6 minutes, or until the mushrooms have released their moisture and the liquid has evaporated.
Stirring constantly, sprinkle in the flour and cook for 1 minute. Stir in broth and, still stirring constantly, bring to a simmer. Add cream, reduce heat to medium, and continue simmering about 12 minutes, until the sauce has thickened and reduced to 3½ cups. Taste and adjust seasoning as needed.
Place green beans back into the Dutch oven and stir to evenly coat them with sauce. Pour into a 3 quart baking dish. Evenly spread topping over the beans and bake for about 15 minutes, until the sauce is bubbling and topping is golden brown. Serve immediately.