1. Preheat the oven to 180˚C, gas mark 4. For the pastry, pulse the flour, butter, sugar and salt in a food processor until combined. Continue to process while adding the eggs and egg yolk, stopping as soon as it comes together. If the mixture looks a little dry, add 1-2 tablespoons of ice-cold water.
2. Roll out on a lightly floured surface to a large circle, the thickness of a £1 coin. Put a loose-bottomed tart tin (about 22-23cm x 3-4cm) onto a baking sheet and line with the rolled pastry, pressing it right into the corners. Leave the edges overhanging. Keep a little ball of raw dough in case you need to patch-up any cracks after baking. Prick the base with a fork and chill for 30 minutes.
3. Line with parchment and baking beans (or rice/pulses). Bake for 25 minutes until the edges are golden brown. Remove the baking beans and fill any cracks with some reserved dough, if needed. Brush the inside with beaten egg white and bake for a further 10-15 minutes until golden brown. Cool for a few minutes and then trim off the overhanging pastry with a serrated knife. Turn the oven up to 200˚C, gas mark 6.
4. Meanwhile, make the curd. Finely grate the lemons and squeeze to give 200ml juice (make the quantity up with water if necessary). Place into a bowl with the eggs, egg yolks, butter, caster sugar and a pinch of salt. Set over a pan of barely simmering water and whisk continuously until the butter has melted. Cook gently, for 5-7 minutes, stirring all the time until the curd thickens. Sieve the warm curd into the pastry case and bake for 15-17 minutes, until just set. Cool for 30 minutes and then transfer to a wire rack to cool completely. Serve chilled or at room temperature with some lightly whipped cream.