Add the grated lemon or lime rind to the cream in a large bowl and whisk until stiff. Stir in the dry white wine, then whisk in the lemon or lime juice little by little. Add sugar to taste.
Whisk the egg whites until they form stiff peaks. Fold into the whipped cream mixture. Chill.
Before serving, whisk the posset one more time and spoon into a serving dish or into individual glasses. Decorate with lemon or lime slices.
© 1988 Keith Floyd estate. All rights reserved.