Clean the leek, remove the leaves and wash. Blanch in boiling salted water for 2-3 minutes. Take out with a skimmer, drain and let cool down. Cut the leaves in half lengthwise. Peel and finely chop the onion. Add the onions, egg yolk and breadcrumbs to the mince and season with mustard, salt and pepper. Knead everything together.
Form approx. 15 longish meatballs from the minced dough. Heat the oil in a pan and fry the meatballs in it all around at medium heat for 6-8 minutes. Take them out and let them cool down a bit. Wrap 1-2 leek strips around the meatballs. Pin them with a wooden skewer. Arrange on a plate and serve sprinkled with coloured pepper as desired.