Heat the oil in a large pan and add the onion. Cook for a few minutes until soft butnot browned, then add the carrot and cook for a further 2 minutes, stirring often. Stir in the stock, lentils, tomato purée and curry powder and simmer uncovered for20 minutes. Using a food processor or blender, purée the mixture until smooth. Pour it back intothe pan. The mixture should be thick, but if it isn't, stir in 1 teaspoon of cornflour dissolved in 2 teaspoons of water to thicken.