Wash and dry the leek, cut into fine rings and sauté in the butter. Deglaze with the chicken stock, add the lentils and cook covered until they still have a bite. This takes 7-12 minutes, depending on the age of the lenses.
Then stir in the cream and mustard and bring to the boil briefly. Season to taste with pepper, salt, a little lemon juice and a hint of sugar. Serve sprinkled with parsley.
It is a delicious accompaniment to fish and poultry.