Set lentils to sprout for 3 to 4 days according to instructions.
Cut leek into fine rings, potatoes into slices. Heat oil, sauté leek, add potatoes and clear soup, bring to the boil and continue cooking for 20 minutes.
Blend soup, add whipped cream, season to taste with salt and freshly ground pepper.
Rinse lentil sprouts and heat in soup for 3 minutes.
Serve soup sprinkled with cress.