Preheat the oven to 375 degrees and line a baking sheet with parchment paper.
In a large bowl, combine the ground beef, breadcrumbs, egg, onion, garlic, parsley, oregano, salt, and pepper. Mix until the ingredients are evenly distributed.
Divide the meat mixture into four equal portions. Flatten each portion slightly and place one mozzarella cube in the center.
Fold the meat around the cheese and shape each portion into a small loaf, sealing the edges to keep the cheese inside.
Place the loaves on the prepared baking sheet and bake for 25 to 30 minutes until the internal temperature reaches 160 degrees.
Remove from the oven and let them rest for 5 minutes before serving with marinara sauce.