Special equipment: A 3-quart baking dish
Bring a large pot of salted water to a boil. Add the green beans, and cook until bright green and crisp-tender, about 10 minutes. Drain and shake off any excess moisture; set aside.
Melt the butter in a medium pot over medium heat. Add the onions, and cook, stirring, until soft, about 8 minutes. Add the mushrooms and a pinch of salt, and continue to cook, stirring, until they are soft and most of their liquid has evaporated, about 10 minutes. Sprinkle the flour over the vegetables, and stir until all the flour has been absorbed, about 1 minute. Add the chicken broth, half-and-half, 3/4 teaspoon salt and several grinds of pepper. Bring to a boil, then reduce to a simmer and cook, stirring, until the sauce thickens and coats the back of a spoon, about 5 minutes. Remove the pot from the heat, add the green beans and stir to coat in the sauce. Transfer the mixture to a 3-quart baking dish.
Let the casserole cool to room temperature. Wrap well with plastic wrap and foil, label and date, then freeze for up to 2 weeks.
To serve, preheat the oven to 350 degrees F. Discard the plastic wrap and foil from the frozen casserole. Cover with new foil, and bake until warmed through, about 45 minutes. Uncover, and top with the fried onions. Continue to bake until the casserole is hot and bubbly and the onions have crisped slightly, about 20 minutes more.
You can serve right away instead of freezing. Just top the green bean mixture with fried onions and bake until the casserole is hot and bubbly and the onions have crisped slightly, 25 to 30 minutes.
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