Cook the Lasagna Noodles: Bring a large pot of salted water to a boil. Cook the lasagna noodles according to the package instructions until al dente, then drain and lay them flat on a clean kitchen towel to prevent sticking.
Sauté the Vegetables: In a large skillet, heat the olive oil over medium heat. Add the chopped onion and minced garlic; sauté until the onion is translucent (about 3-4 minutes). Add the sliced mushrooms, and cook for another 5-7 minutes until they are tender and browned. Stir in the chopped spinach, thyme, oregano, and season with salt and pepper. Cook until the spinach wilts (about 2 minutes). Remove from heat and let cool slightly.
Prepare the Ricotta Mixture: In a medium mixing bowl, combine the ricotta cheese, egg, and a pinch of salt and pepper. Mix well until smooth.
Assemble the Lasagna: Preheat your oven to 375°F (190°C). In a greased 9x13 inch baking dish, spread a thin layer of marinara sauce on the bottom. Place three lasagna noodles over the sauce. Spread half of the ricotta mixture over the noodles, followed by half of the sautéed mushroom mixture, then sprinkle with 1 cup of mozzarella cheese. Repeat the layers using three more noodles, the remaining ricotta, the remaining mushroom mixture, and another cup of mozzarella cheese. Finish with the final three noodles topped with marinara sauce, the remaining mozzarella cheese, and grated Parmesan cheese on top.
Bake the Lasagna: Cover the baking dish with aluminum foil (to prevent sticking, spray the foil with cooking spray). Bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 15-20 minutes until the cheese is bubbly and golden brown.
Cool and Serve: Allow the lasagna to cool for about 10-15 minutes before slicing. This helps the layers set and makes it easier to serve.
Notes
Serve warm, garnished with freshly chopped parsley or basil for a pop of color. Pair with a crisp green salad for a complete meal.