Make your favourite pastry, bake and allow to cool.
Add 400 ml water, the lemon juice, the sugar and the citric acid and heat until the sugar has melted completely. Skim off any froth as it forms.
Hydrate the gelatine in 100 ml water and reserve.
Remove the saucepan from the heat and allow to cool to 80 C, then add the hydrated gelatine.
Place the liquid on the baked and cooled pie pastry. Refrigerate for 20 hours.
Remove, garnish as you wish and serve.