To make the meatballs, mix the ground beef, onion, garlic, breadcrumbs, cheese and chopped parsley together in a bowl until combined. Season well and add the beaten egg to bind, mixing with your hands.
Shape a small piece of the mixture into a ball and fry in an oiled pan until cooked, then taste for seasoning - if necessary, adjust the seasoning of the uncooked mixture. With damp hands, shape the mixture into about 16 meatballs, trying not to press them too tightly. Place on a large plate, cover with clingfilm and chill for at least 30 minutes.
To cook the meatballs, heat the olive oil in a large, wide fry pan. Add the chilled meatballs and fry for 5 minutes, turning frequently, until browned all over.