Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish. Slice the zucchini into thin, even slices. You can use a mandoline for this to ensure even thickness.

Heat olive oil in a large skillet over medium heat. Add the chopped onion and cook until softened, about 5 minutes. Add the minced garlic and chopped bell pepper and cook for another 3 minutes. Stir in sliced mushrooms and cook until softened.

Pour in the crushed tomatoes and tomato sauce. Stir in dried oregano, dried basil, salt, and black pepper. Bring to a simmer and cook for 15 minutes, stirring occasionally.

In a mixing bowl, combine the ricotta cheese and half of the Parmesan cheese. Mix well.

Spread a thin layer of tomato sauce on the bottom of the prepared baking dish. Arrange a layer of zucchini slices over the sauce, overlapping slightly. Spread half of the ricotta mixture over the zucchini. Sprinkle with 1/3 of the mozzarella cheese. Pour another layer of sauce over the mozzarella cheese. Repeat layers: zucchini, ricotta mixture, mozzarella, sauce. Top with the remaining zucchini slices, sauce, mozzarella cheese, and Parmesan cheese.

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

Let the lasagna rest for 10-15 minutes before cutting and serving. This helps it hold its shape better.