Remove the vanilla seeds from the pod and put seeds and pod into a small pan. Add the water, sugar and reserved grapefruit juice. Bring to a boil, reduce heat and simmer to make a syrup. Put in the refrigerator until thoroughly chilled.
Toss the prepared fruit in the chilled syrup. Spoon onto plates and pour over the chilled Prosecco. Serve with creme fraiche.
Recipe (c) Jamie Oliver 2002