Preheat the oven to 375°F (190°C).

Boil the lasagna noodles according to package instructions until al dente.

In a large skillet, heat olive oil over medium heat and add diced onion, cooking until translucent, about 3-4 minutes.

Stir in garlic, zucchini, and bell pepper, cooking for another 5 minutes.

Add chopped spinach and marinara sauce, stirring to combine, and simmer for 5-7 minutes until the spinach wilts.

In a mixing bowl, combine ricotta cheese, egg, Italian seasoning, salt, and pepper.

In your baking dish, spread a thin layer of the vegetable sauce on the bottom, layer three noodles, half of the ricotta mixture, and a third of the remaining sauce.

Repeat the layers, finishing with sauce and topping with mozzarella and Parmesan cheese.

Bake for 30-35 minutes until golden and bubbly.

Let the lasagna rest for at least 10 minutes before slicing.