1. Preheat your oven to 375°F (190°C). Line a large baking sheet with parchment paper.
2. In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, egg, garlic, Italian seasoning, salt, and black pepper. Mix gently with your hands until just combined.
3. Take approximately 2 tablespoons of the meat mixture and flatten it slightly in your palm. Place a cube of mozzarella in the center and fold the meat around it, sealing it completely. Repeat with remaining meat and cheese.
4. Heat olive oil in a large skillet over medium heat. Add the meatballs, cooking for about 2 minutes on each side until browned. You might need to do this in batches.
5. Transfer the browned meatballs to the prepared baking sheet. Pour marinara sauce over the top.
6. Bake in the preheated oven for 15-20 minutes, until the meatballs are cooked through and the cheese is melted.
7. Garnish with fresh basil before serving, if desired.

A little tip from my kitchen: make sure your mozzarella cubes are well-sealed within the meatball to prevent any cheese leakage during cooking. This ensures that every bite is as delightful as the first!