Heat 15ml of oil in a pan and fry onions for 3 minutes until soft.
Stir in oregano, celery salt, and cumin then remove from heat to cool.
Mix pork mince, onions, egg yolk, flour, and mustard in a bowl and season.
Roll the mixture into small walnut-sized balls on a floured surface.
Heat 15ml of oil and fry meatballs for 5 minutes until evenly browned.
Check one meatball to ensure the meat is brown and cooked through.
Place meatballs on a dish and keep them warm.
Heat 15ml of olive oil and fry garlic for 1 minute.
Add 250ml of passata, bring to a boil, and season to taste.
Spoon sauce over the meatballs and top with grated cheese.