Wash the grapes, pluck them from the stems and, except for 8 green ones for decoration, cut them in half and remove the seeds. Chop the walnuts coarsely, except for 8 halves. Mix the grapes and chopped walnuts and divide into 4 dessert glasses.
Soak gelatine in cold water. Beat egg yolk and 130 g sugar until creamy. Stir in lemon juice and champagne. Dissolve the gelatine, pour into the cream while stirring constantly and chill until it begins to gel.
Whip cream until stiff and fold in. Spread on the grapes and chill for about 1 hour. Cut remaining grapes in half, moisten a little and roll in remaining sugar. Garnish the cream with the grapes, walnuts and mint.