Preheat the oven to 375°F (190°C).

In a large skillet, heat olive oil over medium heat and sauté onion and garlic until they are soft.

Add the diced bell pepper and zucchini to the skillet, cooking until tender.

Stir in the spinach and cook until wilted, then season with salt, pepper, and oregano.

In a mixing bowl, combine ricotta, half the mozzarella, and half the Parmesan cheese.

Spread a thin layer of marinara sauce on the bottom of a 9x13 inch baking dish.

Layer three lasagna noodles over the sauce, followed by half of the ricotta mixture, half of the sautéed vegetables, and a third of the remaining marinara sauce.

Repeat the layering process with another three noodles, the remaining ricotta mixture, vegetables, and marinara sauce.

Top with the final three noodles, remaining marinara sauce, and the rest of the mozzarella and Parmesan cheeses.

Cover the baking dish with aluminum foil and bake for 25 minutes.

Remove the foil and bake for an additional 15-20 minutes until the cheese is bubbly and golden brown.

Let the lasagna cool for 10 minutes before slicing.