Place the raspberries and sugar in a saucepan and cook over a low heat, with stirring occasionally, until simmering. Cook gently for 5 minutes, then leave to cool. Halve the passion fruits and scoop out the seeds and juice. Pour the raspberries into a blender or food processor, add the passion fruit pulp and blend for a few seconds. Rub the fruit sauce through a fine nylon sieve to remove the seeds.
Fold the strawberries into the sauce, then spoon into four stemmed glasses. Serve with plain finger biscuits.