Heat the oil in a saucepan. Add the onion, cover and fry very gently for about 5 minutes or until the onion is soft but not brown. Stir in the curry powder and cook for a further few moments to bring out the flavour. Stir in the stock, tomato purée, strained lemon juice and the jam or chutney. Stir until boiling and then simmer for 5 minutes. Strain the sauce into a basin and leave until quite cold.
Blend together the curry mixture and the mayonnaise. Stir in the cream just before using.