This rich mousse from Boston's Le Bocage restaurant is very tart and oh, so good!
1 package (1 tablespoon) unflavored gelatin
Zest of 2 lemons, grated fine
Juice of 3 lemons
3 eggs, separated
1/2 cup plus 3 tablespoons sugar
1-1/2 cups heavy cream
Combine gelatin, zest, and juice in a noncorrodible saucepan and heat, stirring, just until gelatin is thoroughly dissolved. Beat egg yolks with 1/2 cup sugar until the mixture is thick and pale, then stir in the slightly cooled gelatin mixture. Chill until thick and almost, but not quite, set. Beat egg whites with the remaining 3 tablespoons sugar until they form stiff but still shiny peaks, then whip the cream and fold everything together. Spoon into stemmed glasses or other decorative serving bowls and chill until set.