Finely chop the onion and coarsely grate the mountain cheese. Mix both with the minced meat, mustard, parsley, salt, eggs, pepper, and breadcrumbs until well combined. Add more breadcrumbs if needed until the mixture is easy to shape.
Shape the mixture into 8 to 10 meatballs using your hands.
Heat oil in a pan over medium-high heat. Fry the meatballs, turning occasionally, until browned on all sides (about 5 minutes).
Reduce heat to low, cover the pan with a lid, and cook for 10 minutes until the meatballs are cooked through (internal temperature 71°C / 160°F).