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Rub the butter into the flour until the mix looks like fine breadcrumbs. Stir in the icing sugar, then make a well and use a round-bladed knife to stir in the oil, egg yolk and 1½-2 tbsp cold water until dough comes together. Without overhandling, gather into a ball. On a lightly floured surface, roll out so it fits a 23 x 2cm loose-bottom flan tin. Ease the pastry into the tin, then trim the edges by rolling the rolling pin over the top. Press the pastry into the flutes so it sits very slightly proud of the edge (this extra height helps in case of any shrinkage). Lightly prick the pastry base with a fork, then chill for about 10 mins. Heat the oven to 190C/170C fan/gas 5.
Meanwhile, make the filling. Beat the eggs and egg whites together with a wooden spoon until well mixed. Sift the icing sugar into a bowl, then gradually beat in the eggs. If the mix is at all lumpy, simply beat with a wire whisk. Stir in the lemon zest and the juice. Leave to stand so the lemon flavour can develop.
Sit the chilled pastry case on a baking sheet. Line with baking parchment and baking beans and bake blind for 20 mins until well set. Carefully lift out the beans and paper, then bake the pastry case for another 3-5 mins until the base is cooked and pale golden.
Strain the lemon mixture through a sieve. Beat the crème fraîche in a medium bowl until smooth, then slowly stir in the lemon mix until well blended. Transfer to a jug, then carefully pour two thirds into the warm pastry case. Place in the oven with the oven shelf half out, pour in the rest of the filling, then carefully slide the shelf back in. Reduce the heat to 150C/130C fan/gas 2. Bake for 25-30 mins until barely set with a slight wobble in the middle. Cool for about 1 hr, then serve with a light dusting of icing sugar. Best eaten the same day.