Put the sausage in boiling water. Leave to stand for about 10 minutes at medium heat. In the meantime drain the lentils. Clean the leek, cut into fine rings, wash and drain. Heat the fat. Steam the leek in it while turning. Add the lentils and heat.
For the sauce, briefly bring cream and stock to the boil. Season to taste with salt, pepper and mustard. Stir in the sauce thickener and bring to the boil again. Peel and slice the sausage. Arrange sausage, lentil vegetables and sauce garnished with parsley on plates. Serve with baguette