In a large bowl, combine ground beef, ground turkey, breadcrumbs, grated Parmesan cheese, egg, minced garlic, oregano, basil, salt, and pepper. Mix until just combined.
Roll the mixture into small meatballs, about 1 inch in diameter, and place them on a baking sheet lined with parchment paper.
Bake the meatballs in the preheated oven for about 20 minutes, or until browned and cooked through.
While the meatballs are baking, heat a skillet over medium heat. Add the chopped bell peppers and black olives, sautéing them for about 5 minutes until they are slightly softened.
In a large casserole dish, spread a thin layer of marinara sauce on the bottom. Add the baked meatballs evenly over the sauce, then layer the sautéed peppers and olives on top. Pour the remaining marinara sauce over the meatballs and vegetables.
Sprinkle shredded mozzarella cheese over the top of the casserole.
Cover the casserole dish with foil and bake for 25 minutes. Remove the foil and bake for another 10-15 minutes, or until the cheese is bubbly and golden.
Remove from the oven and let it cool for a few minutes. Garnish with fresh parsley before serving.
Notes
Feel free to customize the vegetables based on your preference.