In a cup, pour 2 or 3 tablespoons of boiling water over 1 sachet gelatine. Stir thoroughly until dissolved and set aside.
Separate 4 eggs, reserving the whites in a large bowl. Place the yolks in a microwaveable bowl, add the grated rind and juice of one lemon, 4 tablespoons water and a pinch of salt. Microwave on medium for 40 seconds, then stir in the gelatine.
Leave to cool, then stir in 4 level tablespoons of artificial sweetener.
Whisk the egg whites until they form peaks. Whisk in 400g fromage frais, then fold in the lemon mixture using a metal spoon.
Pour into 4 sundae glasses, then chill in the fridge until set.