Cut the pork belly into slices about 1 cm thick. Bring vinegar, water and sugar to a boil. Add bay leaves, mustard seeds and juniper berries and simmer over medium heat for about 5 minutes.
Remove the skin from the onions and cut them into slices. Add to the vinegar broth along with the pork belly. Cook at low temperature for about 1 hour. Remove the meat from the broth and place it in a shallow dish.
Season the gravy with salt and freshly ground pepper and degrease with a fat skimmer. Soak gelatine in cold water, squeeze out and let it melt in the warm gravy. Pour over meat and gelatinize, preferably for one night.
Rinse potatoes and make about 20 min. Peel, cool and cut into slices with an egg slicer. Heat lard in a frying pan, roast potatoes in it in batches. Season with salt, pepper and chopped rosemary. Serve with the sour meat.
Tip: Instead of clarified butter, you can also use butter in most cases.