Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with cooking spray or olive oil.

Fill a large pot with salted water and bring to a boil. Add lasagna noodles, cook until al dente, drain and set aside.

Heat a large skillet over medium heat, add olive oil. Cook onion until translucent, about 3–4 minutes. Add garlic and cook for another minute.

Add mushrooms, thyme, salt, and pepper. Cook until tender and liquid has evaporated.

Add spinach to the skillet and cook until wilted, about 2–3 minutes. Remove from heat.

Ladle marinara sauce onto the bottom of the baking dish. Place three noodles on top.

Spread half of the ricotta over noodles, add half of the mushroom mixture, and sprinkle a third of the mozzarella and Parmesan.

Repeat layers with remaining noodles, ricotta, mushrooms, and cheeses, finishing with marinara sauce on top.

Cover with foil and bake for 25 minutes. Remove foil and bake for another 10–15 minutes until cheese is bubbly and golden.