French Onion Green Bean Casserole

Instructions

1. Caramelize the onions: In a large, wide saucepan, melt 1 tablespoon of the butter over medium heat. Add the onions and cook 5 minutes. Reduce the heat all the way to low and stir in 1 teaspoon salt and 1/2 teaspoon pepper. Continue to cook over super low heat, stirring periodically, until the onions cook down significantly and are golden and caramelized - depending upon your pan, about 90 to 120 minutes. If at any point the onions start to stick, stir in water 1 tablespoon at a time to loosen. Remove to a plate.
2. Place a rack in the center of your oven and preheat to 375°F. Coat an 8x10-in or similar casserole dish with nonstick spray.
3. Bring a very large pot of water to a boil. Add the green beans and cook just until crisp-tender, about 3 minutes. Plunge into an ice bath to stop the cooking, then drain and transfer to the towel. Pat dry.
4. Heat 2 tablespoons butter in a large, wide saucepan over medium heat (if you want to use the one you caramelized the onions in, wipe it out with a paper towel first). Add the mushrooms. Cook until the mushrooms brown, about 10 minutes.
5. Sprinkle the flour over the top. Cook, stirring constantly, for 1 to 2 minutes, until all of the flour turns golden and no white bits remain. The pan will seem very dry.
6. Slowly add the milk a few splashes at a time, stirring between each addition to prevent lumps from forming. Increase the heat to medium high. Cook and stir, allowing the sauce to bubble, running a wooden spoon or spatula along the bottom of the pan until the sauce reduces and thickens to resemble a creamy gravy, about 8 minutes.
7. Stir in the remaining 1 teaspoon salt, 1/2 teaspoon pepper, and nutmeg. If at any point the sauce gets too thick, stir in a little more milk as needed.
8. Spread half of the green beans into the prepared dish. Spoon half of the mushroom sauce over the top, then sprinkle with half the caramelized onions, and half of the cheese. Add the remaining green beans, remaining sauce, and remaining caramelized onions.
9. Melt the remaining 2 tablespoons butter in a small bowl, then stir in the croutons and Parmesan. Sprinkle on top. Sprinkle with the remaining cheese.
10. Bake until the casserole is hot and bubbly and the cheese is melted and the top is lightly browned, about 25 to 30 minutes. Let stand 10 minutes, then serve.