1. Preheat the oven to 200°C/400°F. In a large mixing bowl, combine the ground meat, salt, and black pepper, mixing well with your hands or a wooden spoon.

2. Divide the meat mixture into four equal portions and shape each into a ball. Create a small indentation in the center of each ball, place a mozzarella ball inside, then close the meat around the cheese and shape into a meatball.

3. Place the meatballs on a baking sheet lined with parchment paper, leaving some space between each to allow for even cooking. Drizzle with olive oil and gently roll the meatballs to coat evenly.

4. Bake the meatballs in the preheated oven for 18-20 minutes, or until cooked through.

5. While the meatballs are baking, heat olive oil in a large skillet over medium heat. Add the chopped onions and sauté until translucent, about 5 minutes. Add the minced garlic and cook for an additional minute.

6. Add the canned crushed tomatoes, salt, and black pepper to the skillet. Stir well to combine, then bring the sauce to a simmer. Let cook for 10-15 minutes, stirring occasionally, until the sauce has thickened slightly.

7. Once the meatballs are done, remove them from the oven and carefully place them in the skillet with the tomato sauce. Let simmer for 2-3 minutes to coat the meatballs evenly.

8. Serve the meatballs with a side of spaghetti or linguine, topped with additional fresh basil leaves and grated mozzarella cheese if desired.