Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
Heat 1-2 tablespoons of olive oil in a large skillet over medium heat. Add the finely chopped garlic and sauté until fragrant, about 1 minute. Add the fresh spinach and cook until wilted. Remove from heat and set aside to cool slightly.
In a large mixing bowl, combine the ground beef, ground sausage (or turkey or ground chicken), bread crumbs, eggs, milk, finely chopped garlic, Parmesan cheese, cooked spinach, salt, and pepper. Mix until well combined.
Take a small amount of the meat mixture and flatten it in your hand. Place a cube of mozzarella cheese in the center, then wrap the meat mixture around the cheese to form a ball, ensuring the cheese is completely covered. Repeat with the remaining meat mixture and cheese.
Heat 2 tablespoons of olive oil in a large skillet over medium heat. Brown the meatballs on all sides, working in batches if necessary. Remove the browned meatballs from the skillet and place them on the prepared baking sheet.
Bake the meatballs in the preheated oven for 20-25 minutes, or until fully cooked through.
While the meatballs are baking, heat the marinara sauce in a saucepan over medium heat.
Once the meatballs are cooked, remove them from the oven and pour the warm marinara sauce over them.