Heat olive oil in a large skillet over medium heat. Add onion and garlic and cook until softened, about 5 minutes. Add mushrooms, zucchini, and bell pepper and cook until tender, about 5-7 minutes.
Stir in crushed tomatoes, oregano, salt, and pepper. Bring to a simmer and cook for 15 minutes, stirring occasionally.
In a medium bowl, combine ricotta cheese, Parmesan cheese, and egg.
Spread a thin layer of tomato sauce in the bottom of a 9x13 inch baking dish. Top with 3 lasagna noodles, 0.5 of the ricotta mixture, 0.33 of the mozzarella cheese, and 0.33 of the remaining tomato sauce. Repeat layers twice. Finish with the rest of the mozzarella cheese.
Cover with foil and bake for 25 minutes. Remove foil and bake for another 20 minutes, or until golden brown and bubbly.
Let stand for 10 minutes before serving.
Notes
Feel free to add other vegetables you enjoy, such as spinach or carrots. You can also use different types of cheese, such as provolone or Monterey Jack.