Preheat oven to 375°F (190°C) and lightly grease a 12-cup standard muffin tin.
In a large mixing bowl, combine ground beef, breadcrumbs, milk, egg, onion, garlic, salt, pepper, and oregano.
Mix by hand until ingredients are evenly distributed, being careful not to overwork the meat.
Divide the meat mixture into 12 equal portions.
Press one mozzarella cube into the center of each meat portion and roll into a ball, ensuring the cheese is completely enclosed.
Place one meatloaf ball into each muffin tin cavity.
In a small bowl, whisk together the ketchup, brown sugar, and mustard to create the glaze.
Brush or spoon the glaze evenly over the top of each meatloaf muffin.
Bake for 20 to 25 minutes or until the internal temperature of the meat reaches 160°F (71°C).
Remove from oven and allow to rest in the tin for 5 minutes before serving.