Preheat the oven to 375°F (190°C).
In a skillet, cook the ground turkey or beef over medium heat until browned, breaking it into small pieces as it cooks. Season with salt, pepper, Italian seasoning, and garlic powder. Drain excess fat if necessary.
In a large bowl, combine the cooked meat, diced zucchini, diced bell pepper, and fresh spinach. Mix well.
Pour in the marinara sauce and stir until all ingredients are evenly coated.
Transfer the mixture into a baking dish and spread it out evenly.
Sprinkle the shredded mozzarella cheese on top of the mixture.
Cover the baking dish with aluminum foil and bake for 20 minutes.
Remove the foil and bake for an additional 5 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven and let cool for a few minutes. Garnish with fresh basil if desired, and serve warm.