Best Green Bean Casserole

6 tablespoons butter
1 medium onion, diced small
1 pound button mushrooms, sliced
2 teaspoons sea salt
black pepper to taste
1 1/2 pounds green beans, trimmed, cut into bite-size pieces
¼ cup all-purpose flour
2 cups milk
1 pinch grated nutmeg
1/2 cup grated Parmesan cheese
1/4 cup breadcrumbs
1 can Lars’ Own Crispy Onions*

1. In a large skillet over medium heat, melt 2 tablespoons butter. Add onion, and saute about 5 minutes. Add mushrooms, and cook until most of the liquid has evaporated, about 8 minutes. Season with 1 teaspoon salt and  black pepper. Set aside to cool.
2. Cook beans in boiling water until tender about 8 minutes. Pour into a colander and rinse with cold water. Set aside.
3. Melt the remaining 4 tablespoons butter in a medium saucepan over medium-low heat. Add 1/4 cup flour, and whisk constantly until mixture turns golden, about 2 minutes. Pour in milk, and continue whisking until mixture thickens, about 3 minutes. Stir in nutmeg, and the remaining teaspoon salt and additional pepper. Remove from heat and let cool to room temperature, stirring occasionally. Mix sauce and mushrooms with the beans in a bowl.
4. Pour beans into a greased 9 x 13 inch baking pan. Combine Parmesan and breadcrumbs and sprinkle over casserole.
5. Cover beans with foil and bake at 425°F until heated through, about 30 minutes. Uncover, set oven to broil, and cook until top is golden brown, about 1 minute. Sprinkle fried onions over top and serve immediately.*

*When reheating later, save the fried onions for scattering on top just before serving.

*To make your own fried onions:

4 shallots, cut crosswise into 1/4-inch rings
1/4 cup canola oil
2 tablespoons all-purpose flour

Heat canola oil in a medium skillet over medium-high heat. Toss shallot rings with flour. Fry the shallots in batches, turning frequently, until golden brown. Transfer to paper towels to drain.