In a mixing bowl, combine ricotta cheese with shredded mozzarella, grated Parmesan, a beaten egg, freshly chopped parsley, nutmeg, salt, and pepper. Stir vigorously until smooth and creamy.

Melt ½ tablespoon of butter in a skillet over medium heat, then add diced onion. Sauté for 4-5 minutes until translucent. Toss in baby spinach and crushed red pepper flakes, stirring for another minute until spinach wilts.

In the same skillet, melt 1 tablespoon of butter, add sliced mushrooms, cooking undisturbed for 3-4 minutes until golden brown. Then, add minced garlic and season with salt and pepper. Sauté for an additional minute.

Melt 2½ tablespoons of butter in the same skillet. Whisk in 2 tablespoons of flour, cooking for 1-2 minutes. Gradually add whole milk, whisking until thickened. Season with nutmeg, salt, and pepper, then remove from heat.

Preheat your oven to 375°F (190°C). Grease a 9-inch baking dish.

Spread a thin layer of béchamel sauce in the bottom, layer with noodles, followed by ricotta mixture, sautéed spinach, and mushrooms. Repeat until all ingredients are used, finishing with noodles and béchamel topped with mozzarella and Parmesan.

Cover with foil and bake for 35-40 minutes. Remove foil in the last 10 minutes to crisp the top.

Let the lasagna rest for 10-15 minutes at room temperature before serving, garnished with fresh parsley.