1. Preheat your oven to 400°F (200°C).

2. In a saucepan, combine the crushed tomatoes, oregano, sugar, and a pinch of salt and pepper. Simmer over low heat for 20 minutes, stirring occasionally.

3. Meanwhile, cook the lasagna sheets according to the package directions. Drain and set aside.

4. In a skillet, sauté the chopped onion and minced garlic in butter until soft. Add the sliced mushrooms and cook until they release their liquid and the onion is translucent. Season with salt and pepper.

5. In a large bowl, mix together the cooked mushroom mixture, ricotta cheese, and a pinch of salt and pepper.

6. Spread half of the tomato sauce in the bottom of a baking dish. Lay out 4 lasagna sheets. Divide the mushroom-ricotta mixture among them, leaving a 1-inch (2.5 cm) border. Roll them up tightly and place seam-side down in the dish. Pour the remaining sauce over the top and sprinkle with grated cheese.

7. Dot with butter and bake for 20-25 minutes, until the cheese is melted and bubbling. Serve hot with fresh basil and a green salad. Handle hot baking dish with care to avoid burns.