2sliceshearty sandwich bread, white or wheat, torn into pieces
2tablespoonsunsalted butter, melted
2cupscanned fried onions
10ounceswhite mushrooms, sliced thin
6garlic cloves, minced
1 ¾cupslow-sodium chicken broth
1 ½cupsheavy cream
2poundsfresh green beans, trimmed and cut into 1-inch pieces
For the topping: Pulse bread, butter, and salt in food processor until coarsely ground. Combine bread mixture and canned fried onions in a bowl, transfer to a zipper-lock freezer bag, and freeze. (I placed the topping bag right on top of the casserole in the freezer so I could keep track of both.)
For the casserole: Melt the butter in a large 12-inch skillet over medium heat. Add the mushrooms, salt, and pepper and cook until the mushrooms release their liquid, about 5 minutes. Increase the heat to medium-high and cook until all the liquid has evaporated, about 5 minutes. Add the garlic and thyme and cook until fragrant, about 30 seconds. Stir in the flour and cook until golden, stirring, about 1 minute. Slowly whisk in broth and cream and bring the mixture to a boil. Reduce the heat to medium and simmer, stirring occasionally, until the sauce has thickened, about 10 minutes.
In a large bowl, toss the green beans with the cornstarch until the beans are evenly coated. Transfer the beans to a 9X13-inch baking dish. Pour the warm mushroom mixture evenly over the beans. Let the casserole cool completely on the counter. Once cooled, cover with a layer of plastic wrap and then a layer of foil. Freeze for up to 2 months.
When ready to bake, adjust oven rack to the middle position and heat the oven to 400 degrees. Remove the layer of plastic wrap from the baking dish and replace the foil. Bake, covered, until the sauce is bubbling and the beans are tender, about 80 minutes, stirring the beans thoroughly after 50 minutes. Remove the foil and spread the frozen topping mixture over the beans. Bake until golden brown, about 8-10 minutes. Serve.
Freezable: the topping and casserole can be frozen separately for up to 2 months. Hallelujah!