Cook the lasagna noodles according to package instructions until al dente. Drain and lay flat on a clean towel.
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and garlic, cooking until fragrant and translucent (about 3-4 minutes).
Add the sliced mushrooms to the skillet, cooking until they're browned and their moisture has evaporated (approximately 5 minutes). Stir in the fresh spinach and cook until wilted. Season with oregano, thyme, salt, and pepper. Remove from heat.
In a bowl, mix together the ricotta cheese with half of the grated Parmesan cheese, a pinch of salt, and pepper.
Spread a thin layer of béchamel sauce on the bottom of a 9x13 inch baking dish.
Layer three lasagna noodles on top of the sauce, followed by half of the mushroom mixture. Spoon dollops of the ricotta cheese mixture on top, then sprinkle with 1 cup of shredded mozzarella.
Pour a portion of the béchamel sauce over the top before repeating the layers: noodles, remaining mushroom mixture, dollops of ricotta, and 1 more cup of mozzarella.
Top with the final three noodles, cover with remaining béchamel sauce, and sprinkle the rest of the mozzarella and Parmesan cheese on top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Then, remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden brown.
Let the lasagna cool for about 10 minutes before slicing and serving.
Notes
Serve with a sprinkle of fresh parsley and a side green salad for a complete meal.