Preheat the oven to 375°F (190°C) and lightly grease a 9×13 inch baking dish.
Heat 2 tablespoons olive oil in a large skillet over medium heat.
Add the chopped red bell pepper, chopped zucchini, and chopped yellow onion; cook, stirring occasionally, until the vegetables are tender, about 7–10 minutes.
Add the fresh spinach and cook until wilted, then season the vegetable mixture with salt and pepper to taste. Remove from heat.
Spread a thin layer of marinara sauce (about 1/4 cup) on the bottom of the prepared baking dish.
Place three no‑boil lasagna noodles over the sauce in a single layer.
Spread about one third of the vegetable mixture over the noodles, dollop with about one third of the ricotta cheese, and sprinkle with about one third of the shredded mozzarella.
Repeat the layering twice more (sauce, noodles, vegetables, ricotta, mozzarella), finishing with a final layer of noodles topped with the remaining marinara sauce and the remaining mozzarella cheese.
Cover the dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 20 minutes, until the cheese is bubbly and golden.
Let the lasagna stand for 10 minutes before slicing and serving.
