In a large saute pan, heat olive oil over medium-high heat. Add onions, cook until they soften, about 5 minutes. Add zucchini and peppers. Cook until soft, about 10 minutes.
Add spinach and season with salt and pepper. Cook until wilted, about 2 more minutes. Set the vegetable mixture aside.
In another bowl, mix together ricotta cheese, Parmesan cheese (reserving some for top layer), garlic, and italian seasoning. Season with salt, pepper, and red pepper flakes. Set aside.
To assemble the lasagna, start by spreading a thin layer of sauce on the bottom of a 9×13 pan. Add a layer of lasagna noodles and ricotta mixture. Add a layer of vegetables. Add shredded mozzarella. Repeat until you have used all of the ingredients. Top layer should be sauce with mozzarella and remaining parmesan.
Bake uncovered in a preheated 350 degree oven for 1 hour 20 minutes. Let rest for 20 minutes before slicing.