Melt the gelatin in three and a half tablespoons of water over a double boiler.
In a bowl, take egg yolks and gradually beat in the sugar, lemon juice and lemon rind.
Set the bowl over a pan of simmering water and beat until the mixture is light and thick.
Whisk in the melted gelatin into the lemon mixture and refrigerate it for two minutes.
Fold in the whipped cream into the mixture and pour the mousse into a glass bowl.
Decorate the top with chopped pistachios and refrigerate it until set.