Preheat your oven to 375°F (190°C).
Cook the lasagna noodles according to package instructions and set aside.
In a bowl, mix ricotta cheese, egg, salt, and pepper until well combined.
In a skillet, sauté zucchini, bell pepper, and mushrooms until tender.
Add chopped spinach to the skillet and cook until wilted.
Spread a layer of marinara sauce on the bottom of a baking dish.
Layer 3 lasagna noodles over the sauce.
Spread half of the ricotta mixture, followed by half of the veggie mixture.
Add a layer of mozzarella cheese and repeat the layering process.
Finish with a final layer of noodles, sauce, and mozzarella on top.