Slit nectarine skins with a sharp knife and plunge fruit into boiling water for 30 seconds. Remove with a slotted spoon and peel. Halve and remove stones. Combine sugar with 1/2 cup water in a saucepan and stir over heat, without boiling, until sugar dissolves. Bring to the boil and cook until golden. Remove from heat and, protecting your hand from steam, stir in 1 1/2 cups extra water. Stir over low heat until smooth and well combined, remove from heat, add raspberries and stand for 5 minutes. Process until smooth. Return to saucepan and bring to medium boil.

Reduce heat, add nectarines and simmer for 3-5 minutes, or until lightly poached. Pack nectarines into sterilized jars. Stir Cointreau into raspberry syrup and pour over fruit. Seal while hot and refrigerate for up to 2 weeks before using. Serve at room temperature with chilled mascarpone and crisp wafer biscuits, if desired.

Serves 8.