Preheat the oven to 375 degrees and line a baking sheet with parchment paper.
In a large bowl, combine the ground beef, egg, Parmesan, parsley, oregano, basil, garlic, onion, salt, and pepper. Mix with your hands just until the ingredients are evenly distributed.
Divide the mixture into four equal portions and shape each into a small loaf about 3 inches long.
Place the loaves on the prepared baking sheet and bake for 20 minutes.
Remove the sheet from the oven and spoon marinara sauce over each loaf. Sprinkle the mozzarella on top.
Return to the oven and bake for 10 more minutes, or until the cheese is melted and the internal temperature reaches 160 degrees.