Sift the salt and flour together and pour into the bowl of a food processor (if you don't have one, just pour into a large mixing bowl). Pulse the food processor a few times (or, if not using one, get two butter knives and "cut" the butter into the flour) until the mixture resembles coarse sand.
Sprinkle the sugar over and toss gently until the sugar is well incorporated.
Pour the egg yolks, a little at a time, into the food processor and pulse just until the dough comes together. You may not use all of the egg yolks. Shape the pastry dough into a flat disk and wrap with plastic wrap. Chill for at least 4-5 hours until the pastry is firm (and ideally overnight).
While the pastry dough is chilling, prepare the lemon curd filling. Zest the lemons and squeeze the juice so that you have around ¾ cup of lemon juice.
Put the eggs and the egg yolk into a large bow, add 1 cup of the sugar and whisk together. Add the cream, lemon zest and juice and mix well. Taste the curd and if it's too tart, sprinkle some more sugar over the mixture. Cover and chill the lemon mixture for at least 2-3 hours, preferably overnight.
Line a flan tin with a loose bottomed base with parchment paper. Roll out the pastry dough until it's about 12 inches in diameter (30 cm) and about 3mm (1/16th inch) thick. Gently fold the pastry dough over the flan tin, letting it drape naturally over the sides. Using a flat knife (palette knife), cut away the excess pastry. Cover the pastry in the flan tin with plastic wrap and refrigerate another 30 minutes.
Preheat the oven to 190 degrees Celsius (375 degrees F). Crumple a sheet of parchment paper large enough to cover the entire flan tin use it to line the top of the pastry. Add pie weights and bake the pastry for about 15-20 minutes until the pastry is no longer translucent and just starting to color. Carefully remove the pastry from the oven and discard the parchment paper and pie weights. Return the pastry to the oven to bake for another 3-5 minutes, then remove and let cool slightly.
Reduce the oven to 150 degrees Celsius (300 degrees Fahrenheit)
Put the pastry on a baking sheet. Pour the lemon curd filling into the pastry case until it reaches halfway up the sides, then slide the baking sheet into the oven. Pour more filling into the pastry case until it reaches the top, then carefully close the oven door.
Bake for 40-50 minutes until the lemon curd filling still wobbles but not violently so. Remove from the oven, allowing it to cool, and then carefully remove the tart from flan tin. Dust with powdered sugar and serve.