Wash the knuckle of pork, put it in a pot and add water until the meat is covered. Bring to the boil. Peel the onions and add them to the meat together with the bay leaves, peppercorns and 1/4 litre of vinegar.
Cover and cook over a low heat for about 2 hours. In the meantime, peel and wash the carrot and add it to the meat 5 minutes before the end of the cooking time. Drain mustard gherkins and silverskin onions. Cut the gherkins into quarters.
Remove the carrot from the stock and cut into slices with a colored knife. Remove the knuckle of pork and let it cool down. Remove meat from rind, fat and bone and cut into large pieces. Soak gelatine in cold water.
Measure 3/8 litres of meat stock, pour through a filter bag or a sieve lined with kitchen paper. Bring the degreased broth, remaining vinegar, mustard seeds and sugar to the boil. Season to taste with salt and pepper.
Squeeze out the gelatine and dissolve in the broth and allow to cool slightly. Pour a mirror on the bottom of a box mould (26 cm long) with jelly liquid and allow to set in the fridge.
Wash the parsley and pluck the leaves. Put some carrot slices and parsley leaves on the jelly mirror. Layer the meat, silver onions, gherkins, carrot slices and parsley in the mould.
Pour jelly liquid over it. Leave the knuckle of pork in the refrigerator for a few hours (preferably overnight). Place the mould briefly in hot water and turn it over. Line a plate with lettuce leaves and place the knuckle of pork in aspic on top.
Garnish with bay leaves and parsley. Fried potatoes and mustard moulade taste good with it.