Peel the potatoes and cut into thick pieces. Clean and wash the leek and carrots. Cut the leek into 1/2 cm thick rings and the carrots into finger-thick cubes. Put everything in a saucepan. Cut the sausages diagonally and place them on the vegetables. Pour the meat stock, bring to the boil and then simmer on the lowest setting for about 20 minutes. Stir in the lentils and let cook, covered, for another 20 minutes. Season with pepper. Mix the cream and mustard together and stir into the stew just before serving. Sprinkle with cress on top.