In a large bowl, combine ground beef, egg, breadcrumbs, Parmesan cheese, milk, garlic powder, Italian seasoning, salt, and pepper. Mix until just combined.
Form the mixture into 1 1/2-inch meatballs.
Heat olive oil in a large skillet over medium heat. Add meatballs and cook until browned on all sides, about 10 minutes.
Pour marinara sauce over the meatballs. Reduce heat to low, cover, and simmer for 10 minutes.
Sprinkle shredded mozzarella cheese over the meatballs. Cover and cook until cheese is melted, about 3 minutes.