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 and cook until tender, about 7–10 minutes.
Add the fresh spinach to the skillet and cook until wilted; season the vegetable mixture with salt and pepper to taste.
Spread a thin layer of marinara sauce over the bottom of the prepared baking dish.
Place three no-boil lasagna noodles over the sauce, then spread about one-third of the vegetable mixture over the noodles, dollop one-third of the ricotta, and sprinkle about one-third of the shredded mozzarella.
Repeat the layers (sauce, three noodles, vegetables, ricotta, mozzarella) two more times, finishing with a top layer of noodles, 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, or until the cheese is bubbly and golden.
Let the lasagna stand for 10 minutes before slicing and serving.