To make the pastry, sift the flour and salt into a large bowl. Add the butter and use your fingertips to rub it into the flour until the mixture resembles breadcrumbs. Stir in the sugar, then the egg yolk and 1–1½ tablespoons of cold water. Use a round-bladed knife to stir the mixture until bound together. Knead gently on a floured work surface for a few seconds until smooth. Wrap in plastic wrap and leave to chill in the refrigerator for 20 minutes before using.
Put a baking tray in the oven and preheat the oven to 190°C. Roll out the pastry on a lightly floured work surface to a thickness of 2 mm and use to line a loose-based 24 cm flan tin. Prick the base all over with a fork and chill for a further 10 minutes. Line the pastry case with baking paper and baking beads or uncooked rice and bake on the hot baking tray for 12–15 minutes until the pastry is set and golden.
Meanwhile, make the filling. Mix together the lemon zest and juice, melted butter, eggs, sugar, flour and cornflour in a large bowl and beat until smooth.
Remove the paper and beads or rice from the pastry case and reduce the temperature to 180°C. Pour in the filling and bake for 15 minutes, or until the filling has set. Leave to cool for 10 minutes before removing from the tin. Serve warm or cold, with cream or crème fraîche, if you like.