To prepare the watermelon sorbet recipe put the sugar, water and lemon juice in a pan and slowly bring to the boil, stirring, until the sugar dissolves.
Put the melon, nectarine and a little rosé in a blender and blend the fruit to a fine purée. Add to the sugar syrup with the remaining rosé, stir well and leave to cool.
Whisk the egg white until stiff, adding the icing sugar. Fold into the fruit mixture and finish in an ice cream maker. Alternatively, put the watermelon sorbet in the freezer and freeze for at least 3 hours, stirring thoroughly every 30 minutes.