Arrange all ingredients for use.
Soak the bun in a bowl of lukewarm water.
Peel and finely dice the onions.
Wash the parsley, shake dry and finely chop the leaves.
Place the minced meat in a bowl with the egg, mustard and onions. Season with salt and pepper.
Add the squeezed bun and the parsley and mix together well.
Shape into 8 meatballs.
Heat the oil in a large pan and fry the meatballs for 1-2 minutes until browned. Turn and brown on the other side for another 1-2 minutes. Reduce the heat and add the butter to the pan.
Cook the meatballs for a further 4-5 minutes until cooked through, turning occasionally and spooning over the butter. Drain on kitchen paper and serve.