Prepare the Mushroom and Spinach Filling: Heat olive oil in a large skillet over medium heat. Add onion and cook until softened, about 5 minutes. Add garlic and cook for 1 minute more. Add sliced mushrooms and cook until they release their liquid and begin to brown, about 8-10 minutes. Add spinach and red pepper flakes (if using) and cook until spinach is wilted, about 2-3 minutes. Season with salt and pepper to taste. Drain any excess liquid from the skillet. Set aside.

Prepare the Cheese Mixture: In a medium bowl, combine ricotta cheese, Parmesan cheese, basil, and egg. Season with salt and pepper to taste. Mix well.

Prepare the Tomato Sauce: Heat olive oil in a saucepan over medium heat. Add garlic and cook for 1 minute. Add crushed tomatoes, tomato sauce, oregano, basil, and sugar (if using). Season with salt and pepper to taste. Bring to a simmer and cook for 15-20 minutes, stirring occasionally, until slightly thickened.

Assemble the Lasagna: Preheat oven to 375°F (190°C). Spread a thin layer of tomato sauce in the bottom of a 9x13 inch baking dish. Place a layer of lasagna noodles over the sauce, overlapping as needed to cover the bottom of the dish. Spread half of the ricotta cheese mixture over the noodles. Top with half of the mushroom and spinach filling. Sprinkle with 1/3 of the mozzarella cheese. Repeat layers: noodles, remaining ricotta cheese mixture, remaining mushroom and spinach filling, 1/3 of the mozzarella cheese. Top with a final layer of noodles, remaining tomato sauce, and remaining mozzarella cheese.

Bake the Lasagna: Cover the baking dish with aluminum foil. Bake for 30 minutes. Remove the foil and bake for an additional 15-20 minutes, or until the cheese is melted and bubbly and the lasagna is heated through.

Rest the Lasagna: Let the lasagna rest for 10-15 minutes before slicing and serving. This allows the lasagna to set and makes it easier to cut.