In a large bowl whisk the egg yolks; gradually add the sugar until it starts to look pale and fluffy. Add the juice and rind of the lemons and beat well. In a separate bowl whip the double cream until it becomes thick but not too stiff. Fold it into the egg mixture along with the Greek yogurt.
Whisk the egg whites in a separate bowl with an electric whisk until they are stiff. Fold the egg whites into the yogurt mixture.
Grease a loaf tin and line the bottom with half of the crushed biscuits. Pour all of the mousse mixture over the top of the biscuits. Cover the top of the mousse with the remaining biscuits and sprinkle over some pistachios to decorate.
Cover the tin with cling film and put in the freezer for at least 12 hours.
Take it out of the freezer about 30 minutes before you want to serve, it can be removed from the tin easily. Remove it from the tin by putting a knife around the outside to loosen and serve on a serving platter.
This dessert can be made in ramekins for individual portions. You can make this dessert in advance and will keep for up to 3 months in the freezer.
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