Prepare the Meatball Mixture:
In a large mixing bowl, combine the ground beef, egg, diced onion, minced garlic, fresh parsley, Panko breadcrumbs, Parmesan cheese, dried basil, dried oregano, salt, and pepper. Mix thoroughly to ensure everything is evenly incorporated.

Stuff the Meatballs:
Cut the fresh mozzarella into ½-inch cubes.
Take about 2 tablespoons of the meatball mixture, flatten it with your fingers, place a mozzarella cube in the middle, and carefully seal the mixture around the cheese to form a ball. Repeat to make about 12 meatballs.

Cook the Meatballs:
Heat olive oil in a large non-stick pan over medium-high heat. Add the meatballs and brown them on all sides, carefully flipping them to avoid breaking. Once the meatballs are browned, add the marinara sauce, covering all the meatballs with sauce.

Simmer:
Cover the pan and let the meatballs simmer in the marinara sauce for about 10 minutes, flipping them halfway through to ensure they cook evenly and the cheese inside melts.

Serve:
Serve the mozzarella stuffed meatballs over cooked pasta. Top with fresh parsley and grated Parmesan cheese for extra flavor.