Preheat your oven to 375°F (190°C).
In a large skillet, heat olive oil over medium heat.
Add the chopped onion and minced garlic; sauté until fragrant and translucent.
Stir in the spinach, zucchini, and bell pepper; cook until softened.
Remove from heat and stir in oregano, salt, and pepper to taste.
Spread a thin layer of marinara sauce on the bottom of a 9x13 inch baking dish.
Layer four lasagna noodles across the sauce, followed by a portion of the vegetable mixture.
Add half of the ricotta cheese, half of the mozzarella, and a layer of marinara sauce.
Repeat with another layer of noodles, remaining vegetable mixture, ricotta, mozzarella, and sauce.
Top with the final layer of noodles, remaining marinara, and sprinkle with Parmesan cheese.
Cover with foil and bake for 25 minutes, then remove the foil to bake an additional 15 minutes.
Remove from oven and let sit for 10-15 minutes before slicing.