Preparation

• Thaw frozen spinach and squeeze out excess liquid.
• Cook lasagna noodles until al dente. Drain, rinse under cold water and lay flat on parchment paper with some olive oil.
• In a large skillet over medium-high heat, melt butter, add onions, salt, and pepper. Sauté for 2-3 minutes, then add garlic and cook for another minute. Add chopped mushrooms and Italian seasoning; cook until liquid evaporates, about 7-10 minutes. Stir in drained spinach.
• In a large bowl, combine ricotta, Parmesan, half of the mozzarella, eggs, and parsley. Once spinach mixture cools, combine with ricotta.
• In a saucepan, melt butter over medium heat, add flour to make a roux, then gradually add milk while whisking. Cook until thickened, add Parmesan, nutmeg, salt, and pepper.
• Preheat the oven to 375°F.
• Pour about ½ cup white sauce into a 9x13-inch pan. Layer noodles, ⅓ of spinach mixture, and ½ cup white sauce. Repeat two more times.
• Finish with noodles, remaining white sauce, and top with remaining mozzarella.
• Cover with greased foil and bake for 30 minutes. Remove foil and bake for another 10 minutes until tops are browned.
• Cool for at least 15 minutes before serving.