Shell the prawns and put them aside. Melt the butter in a saucepan, fry the chopped onion without browning, then add the curry powder and flour and fry slowly for at least 20 min. Add the stock, coarsely-chopped apple, coconut and a little salt. Simmer gently for ½ hr., then strain and return to the saucepan. Season to taste, add the lemon juice, put in the prawns and when thoroughly hot serve with well-boiled rice.