Beat the sour cream well with the mayonnaise. Peel and core the apple and cut it into very, very small cubes, and also very finely dice the onion. Pour into the sauce and stir in the sherry. Add lemon juice, sugar, salt, pepper and plenty of curry powder, stir well.
Let it steep in the refrigerator for a good 1/2 hour, then season again to taste.
Particularly good with poultry and as a fondue sauce.