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 half 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 1.5 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.