For the topping:
- 2 medium onions, thinly sliced
- 1/4 cup all-purpose flour
- 2 tablespoons panko bread crumbs
- 1 teaspoon kosher salt
- Nonstick cooking spray
For beans and sauce:
- 2 tablespoons plus 1 teaspoon kosher salt, divided
- 1 pound fresh green beans, rinsed, trimmed and halved
- 2 tablespoons unsalted butter
- 12 ounces mushrooms, trimmed and cut into 1/2-inch pieces
- 1/2 teaspoon freshly ground black pepper
- 2 cloves garlic, minced
- 1/4 teaspoon freshly ground nutmeg
- 2 tablespoons all-purpose flour
- 1 cup chicken broth
- 1 cup half-and-half
- Preheat the oven to 475 degrees F.
- Combine onions, flour, panko and salt in a large mixing bowl. Coat a sheet pan with nonstick cooking spray and spread the onions on the pan. Bake until golden brown, about 30 minutes. Toss the onions 2 to 3 times during cooking. Remove from the oven and set aside. Turn the oven down to 400 degrees F.
- Bring a gallon of water and 2 tablespoons of salt to a boil in an 8-quart saucepan. Add the beans and blanch for 5 minutes. Drain in a colander and immediately plunge the beans into a large bowl of ice water to stop the cooking. Drain and set aside.
- Melt the butter in a 12-inch cast iron skillet set over medium-high heat. Add mushrooms, 1 teaspoon salt and pepper and cook, stirring occasionally, approximately 4 to 5 minutes. Add the garlic and nutmeg and continue to cook for another 1 to 2 minutes. Sprinkle the flour over the mixture and stir to combine. Cook for 1 minute. Add the broth and simmer for 1 minute. Decrease heat to medium-low and add the half-and-half. Cook until the mixture thickens, stirring occasionally, approximately 6 to 8 minutes.
- Remove from the heat and stir in 1/4 of the onions and all of the green beans. Top with the remaining onions. Place into the oven and bake until bubbly, approximately 15 minutes. Remove and serve immediately.