Melt the butter in a saucepan and sauté the leek for about 5 minutes. Add the carrots and continue to cook for 2-3 minutes. Add the lentils, pour over the stock, bring to boil, then cover and simmer for 10 minutes. Add the cauliflower and apple and continue to cook for 15 minutes or until lentils and vegetables are tender. Puree.