In a large bowl, mix together the ground beef, minced garlic, shredded mozzarella, grated Parmesan, chopped parsley, beaten egg, salt, and pepper. Shape the mixture into 16 meatballs.

Heat olive oil in a large skillet over medium heat. Add the meatballs and cook, turning them occasionally, until they are browned on all sides, about 10 minutes. Remove the meatballs from the skillet and place them on a paper towel-lined plate.

In the same skillet, add the chopped onion and cook until softened, about 5 minutes. Add the minced garlic and cook for an additional minute until fragrant. Stir in the crushed tomatoes and dried oregano, then season with kosher salt and black pepper.

Return the meatballs to the skillet, cover, and let simmer until the sauce has thickened, approximately 15 minutes. Top with extra Parmesan cheese before serving.