1.Place the lemon juice, oil, white wine, salt and pepper in a saucepan and bring to the boil and simmer for ten minutes.
2.Add the mushrooms to the pan, bring back to the boil and simmer for five minutes.
3.Remove the mushrooms with a slotted spoon and set aside.
4.Boil the sauce until it is reduced by half.
5.Leave to go cold.
6.Meanwhile cut the cucumber into small pieces.
7.Stir into the sauce with the mushrooms, tomatoes and olives.
8.Chill for at least 12 hours or overnight in the refrigerator.
9.Add the prawns just before serving.