Preheat your oven to 375°F (190°C). Cook the lasagna noodles until al dente, about 8–10 minutes, then drain.

Heat olive oil in a large skillet over medium heat. Add diced onion and minced garlic; sauté until onion is translucent.

Add zucchini, red and yellow bell peppers, and mushrooms (if using) to skillet and sauté until softened, about 5–7 minutes.

Stir in fresh spinach until wilted. Add marinara sauce, dried basil, and oregano; let simmer for 5 minutes.

Combine ricotta cheese, beaten egg, salt, and pepper in a mixing bowl until smooth.

In a baking dish, spread a thin layer of sauce, place 3 noodles, top with half of the ricotta mixture, half of the sauce, and 1 cup mozzarella.

Repeat with remaining noodles, ricotta filling, and sauce, topping with remaining mozzarella and parmesan.

Cover with foil and bake for 25 minutes, then uncover and bake for an additional 15 minutes or until cheese is golden brown.

Remove from oven and let cool for 10 minutes before slicing and serving.