Heat olive oil in a large skillet over medium heat. Add chopped onion and cook until soft and translucent, about 5–6 minutes.

Add garlic and sauté for about a minute until fragrant. Then add sliced mushrooms and cook patiently. Let them release their moisture and cook it off completely—this step is essential for flavor and to avoid a watery lasagna.

Once the mushrooms are golden and slightly caramelized, add spinach and cook until wilted. Season with salt, pepper, and oregano. Set aside to cool slightly.

In a saucepan, heat tomato sauce with basil and chili flakes. Let it simmer gently for 10–15 minutes. This deepens the flavor and makes the sauce taste more homemade, even if you’re using store-bought.

In a mixing bowl, combine ricotta, mozzarella, parmesan, and egg. Mix until smooth and creamy. This layer adds richness and helps bind everything together.

Preheat oven to 180°C (350°F).

Spread a thin layer of tomato sauce at the bottom of your baking dish. Add a layer of lasagna sheets, followed by mushroom-spinach filling, cheese mixture, and more sauce.

Repeat the layers, keeping them even and balanced. Finish with a generous layer of sauce and a sprinkle of cheese on top.

Cover the dish with foil and bake for 30 minutes. Then remove the foil and bake for another 15 minutes until the top is golden and bubbling.

Let the lasagna rest for 10–15 minutes before slicing. This helps it set and makes serving much easier.