1. Preheat your oven to 375°F (190°C). Lightly grease a 9×9-inch baking dish with olive oil.
2. Heat olive oil in a large skillet over medium heat. Add chopped onion and sauté until translucent, about 5 minutes.
3. Add minced garlic and cook for another minute until fragrant. Stir in the chopped spinach and cook until wilted, about 2 minutes. Season with a pinch of salt and pepper. Remove from heat and set aside.
4. In the same skillet, lightly sauté zucchini, eggplant, and bell pepper slices separately for 2-3 minutes each, just until they start to soften but still hold their shape. This step helps reduce excess moisture in the final dish.
5. Spread a thin layer of marinara sauce on the bottom of your baking dish. Layer half of the zucchini, eggplant, and bell pepper slices over the sauce.
6. Dollop half of the ricotta cheese evenly over the veggies, then sprinkle half of the sautéed spinach and onions on top.
7. Sprinkle ½ cup of shredded mozzarella and a pinch of dried oregano and basil over the layer.
8. Repeat the layers: marinara sauce, remaining veggies, ricotta, spinach-onion mix, mozzarella, and finish with the grated Parmesan cheese on top.
9. Cover the dish loosely with foil and bake in the preheated oven for 25 minutes.
10. Remove the foil and bake for an additional 15 minutes until the cheese is bubbly and golden.
11. Let the dish rest for 10 minutes before serving. This helps the layers settle and makes slicing easier.

From my experience, letting the dish rest is a game-changer—especially when you’ve got little ones tugging at your pants and asking if dinner is ready yet. It gives you a moment to breathe and keeps the layers intact when serving.