Peel and chop the onion. Clean, wash and roughly cut the leek into pieces. Peel, wash and roughly chop the carrot and celery. Put onion, vegetables, whole slices of ham, bay leaf, peppercorns and 1 l water in a large pot.
Boil up covered and simmer for about 20 minutes.
Boil the lentils in water, cover and simmer for about 30 minutes. Then drain.
Wash parsley, shake dry and chop finely. Remove the ham from the stock and cut into fine short strips. Pour stock through a sieve and collect.
Heat butter in a pot. Dust with flour and sauté until light brown. Deglaze with stock and cream and bring to the boil while stirring. Simmer for about 5 minutes. Stir in mustard. Season to taste with salt and pepper.
In the meantime heat oil in a pan. Fry the ham strips in it until crispy. Stir in lentils and heat briefly. Fold in parsley. Divide the soup into four deep plates, add the lentil and ham mix and serve.