Preheat the Oven: Preheat your oven to 375°F (190°C). Meanwhile, lightly grease a large casserole dish with olive oil.
Prepare the Vegetables: In a large bowl, combine the onion, garlic, zucchini, bell pepper, and spinach. Add a pinch of salt and pepper, and toss until the vegetables are evenly coated.
Layer the Base: Dump the mixed vegetables into the casserole dish, creating an even layer. Next, pour the tomato sauce evenly over the vegetables. Then, scatter the diced tomatoes on top.
Form and Add Meatballs: In a separate bowl, mix the ground turkey with basil, oregano, smoked paprika, salt, and pepper. Form the mixture into small meatballs. Gently place the meatballs over the vegetable base, ensuring even distribution.
Add Cheese: Sprinkle the shredded mozzarella evenly over the top.
Bake: Place the casserole dish in the preheated oven. Bake for 35 to 40 minutes, or until the meatballs are cooked through and the cheese is melted and lightly golden. If necessary, cover with foil for the first half of baking to prevent the cheese from browning too quickly.
Serve: Take the casserole out of the oven and allow it to settle for a few minutes before serving. Then, serve warm, and enjoy a comforting meal that is both quick and easy to prepare.