Instant Pot Spinach and Mushroom Lasagna

Name: Instant Pot Spinach and Mushroom Lasagna
Ingredients: 4 tablespoons unsalted butter; 1 pound button mushrooms, sliced; Kosher salt; 1/4 cup all-purpose flour; 1/4 teaspoon freshly grated nutmeg; 2 cups whole milk; 20 ounces frozen spinach, thawed and squeezed dry; 1 cup chopped fresh basil; 1/2 cup whole-milk ricotta; 1/2 teaspoon crushed red pepper flakes; 1 clove garlic, finely grated; 1 1/4 cups grated Parmesan; One 24-ounce jar marinara sauce; One 9-ounce box no-boil lasagna noodles; 1 cup grated mozzarella