Spinach Lasagna with Portobellos and Béchamel

Name: Spinach Lasagna with Portobellos and Béchamel
Ingredients: 6 tablespoons butter, divided; 6 large portobello mushrooms; 1 large yellow onion, finely diced; 6 cloves garlic, chopped; 20 ounces frozen spinach (not chopped is preferable, but chopped is fine); 2 large eggs; 32 ounces whole-milk ricotta cheese; 2 cups Parmigiano-Reggiano cheese, divided; ¼ teaspoon nutmeg, divided; 4 tablespoons all-purpose flour; 5 cups whole milk; 9 ounces no-boil lasagna noodles; Fine sea salt and freshly ground black pepper; Marinara sauce for serving, optional