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

Bring a large pot of salted water to a boil. Cook the lasagna noodles until al dente (follow package timing). Drain and lay flat on parchment or lightly oiled tray.

Heat olive oil in a large skillet over medium heat. Add chopped onion and sauté until soft and translucent (3–4 minutes).

Add minced garlic and cook until fragrant (30–60 seconds).

Add sliced mushrooms and dried thyme. Season with salt and pepper. Sauté until mushrooms release liquid and begin to brown (6–8 minutes).

Stir in chopped spinach and cook just until wilted (1–2 minutes). Taste and adjust seasoning. Remove from heat and let filling cool slightly.

Spoon a thin layer (about 1/2 cup) of marinara sauce into the bottom of the prepared baking dish.

Arrange 3 lasagna noodles over the sauce. Spread half the ricotta over the noodles. Top with half the mushroom–spinach mixture, one-third of the mozzarella, and a little Parmesan.

Repeat with another 3 noodles, remaining ricotta, remaining mushroom mixture, another third of mozzarella, and Parmesan.

Finish with the final 3 noodles, remaining marinara sauce, and the rest of mozzarella and Parmesan.

Cover dish tightly with foil and bake covered for 25 minutes.

Remove foil and bake uncovered for 10–15 minutes until cheese is golden and bubbly.

Let lasagna rest for 10–15 minutes before slicing. Garnish with fresh basil and serve warm.