Cut the pineapples carefully in half lengthways, leaving their green tops attached.

Cut out the pineapple flesh carefully, leaving a 5 mm ( 1/4 inch) border of flesh on the inside of the shell. Remove the cores and cut the flesh into bite-sized pieces.

Put the chopped pineapple into a bowl, along with the prawns, cashew nuts and celery. Pour in the lemon juice and mix well. Divide the mixture equally between the pineapple shells, and chill them in the refrigerator.

In a heat-proof bowl, whisk together the egg and sugar. Stand the bowl over a pan of simmering water, and whisk in the vinegar and tarragon. Continue whisking until the mixture has thickened.

Remove the bowl from the heat and allow to cool completely, whisking occasionally.

When completely cold, whip the cream until it is just beginning to thicken, then fold it into the dressing mixture.

Pour the cream dressing over the salad in the pineapple shells and serve.

Whisking the egg and sugar dressing can be done with an electric mixer. It will not then be necessary to whisk the dressing over a pan of hot water.