Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish with olive oil.

Cook the lasagna noodles in a large pot of salted boiling water until al dente, about 8-10 minutes, then drain and set aside.

In a large skillet, heat olive oil over medium heat, add chopped onion and sauté until translucent.

Add minced garlic and cook for an additional minute, then add sliced mushrooms, dried thyme, salt, and pepper; cook until mushrooms are tender.

Stir in fresh spinach and cook until wilted.

Spread a thin layer of marinara sauce on the bottom of the greased baking dish, place three lasagna noodles on top.

Spread half of the ricotta cheese over the noodles, followed by half of the mushroom and spinach mixture, and top with a third of the mozzarella and Parmesan cheeses.

Repeat the layering process with remaining ingredients and finish with a final layer of noodles, topped with marinara sauce and remaining cheeses.

Cover with foil and bake for 25 minutes; uncover and bake an additional 10-15 minutes until cheese is golden brown.

Let cool for 10 minutes before serving.