In a saucepan, heat olive oil and sauté onion and garlic until softened. Add diced tomatoes, tomato puree, oregano, salt, and pepper. Simmer for 5 minutes.
In a separate skillet, sauté mushrooms until tender. Add spinach and cook until wilted.
Preheat oven to 375°F (190°C).
Assemble the lasagna: In a baking dish, layer lasagna sheets, tomato sauce, mushroom and spinach mixture, and ricotta cheese. Repeat layers until all ingredients are used, ending with a layer of lasagna sheets.
Top with mozzarella cheese.
Bake for 30-35 minutes, or until bubbly and golden brown.