Put the sugar, water and vanilla pod in a large, heavy-based saucepan and heat gently, stirring, until the sugar has dissolved. Bring to a low simmer, add the fruit and cover with a circle of greaseproof or baking parchment to hold the fruit in the syrup. Simmer for 2 minutes then turn off the heat and leave to cool.
Remove the fruit from the liquid with a slotted spoon, reserving the poaching liquid. Peel the skins from the fruit, then cut them in half and remove the stones.
Put 250 ml (8 fl oz) of the poaching liquid in a small, heavy-based saucepan and heat to reduce it for 6—8 minutes until it has a syrupy consistency. Put the fruit in a large bowl, pour over the syrup and toss gently. Arrange the fruit on serving plates, add a spoonful of mascarpone to each one and sprinkle with crushed ginger biscuits. Decorate with a vanilla pod if liked.