Place flour, butter and icing sugar in food processor; process until mixture resembles bread crumbs. Add egg yolk and 1 to 2 tablespoons cold water, and process until mixture forms a dough. Mold dough into 2 flat shapes, wrap in cling film and chill for 20 minutes.
Roll out pastry to 3mm thick and line a 24 cm fluted tart tin; feeze for 20 minutes. Preheat oven to 175 degrees C.
Line pastry with greaseproof paper or foil, fill the foil with dried beans or ceramic baking beans and bake in preheated oven for 15 minutes. Remove foil and beans, and continue baking until pastry is a light golden colour.
In a mixing bowl, mix together eggs, caster sugar, lemon zest, lemon juice and cream. Do not over beat. Strain. Place tart on a baking tray and carefully pour lemon mixture into baked pastry case.
Bake for 20 minutes. The filling will be runny in the centre and will set more as it cools; don't over cook, or cracks will appear. Cool and serve.