In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, parsley, egg, garlic, Italian seasoning, salt, and pepper.
Mix until just combined, being careful not to overwork the mixture.
Form the mixture into meatballs, about 1 inch in diameter.
Place the meatballs in the crockpot.
Pour marinara sauce over the meatballs, ensuring they are well coated.
Cover the crockpot with the lid.
Set the crockpot to low and cook for 6-8 hours or high for 3-4 hours.
About 30 minutes before serving, sprinkle mozzarella cheese on top of the meatballs.
Cover again and let the cheese melt.
Serve warm with your choice of pasta or bread.