Preheat the oven to 375°F. Heat the olive oil in a skillet over medium heat.
Add the onion, garlic, and red bell pepper to the skillet. Cook until the onion turns soft.
Stir in the zucchini slices and cook for another two minutes. Season with salt and pepper, then remove the skillet from heat.
Mix the fresh spinach into the ricotta cheese in a bowl until evenly combined.
Spread a thin layer of marinara sauce in the bottom of an 8x8 baking dish. Add two noodles, then half the ricotta spinach mixture, half the cooked vegetables, and a third of the mozzarella.
Repeat the layers once more, ending with the final two noodles, remaining marinara, and the rest of the mozzarella and parmesan on top.
Cover the dish with foil and bake for 25 minutes. Remove the foil and bake for 10 more minutes until the cheese browns lightly.