FOR THE SUGAR CRUST:
Mix the flour, powdered sugar and fleur de sel in a food processor to blend. Add the butter. Pulse until the mixture resembles a coarse meal.
Drizzle 2 tablespoons of the beaten egg over the dough; discard the remaining egg. Pulse again, just until the dough begins to form.
Gather the dough into a ball; flatten into a disk. Wrap the dough in plastic; chill at least 30 minutes and up to 1 day.
Preheat the oven to 350º F. Roll out the dough on a lightly floured surface to a 12½-inch-diameter round. Transfer to a 10-inch-diameter tart pan with a removable bottom.
Gently press the dough in the pan to cover completely. Press any overhanging dough against the top edge of the pan to trim. Cover and refrigerate the crust until it is cold, about 10 minutes.
Line the crust with foil and fill with pie weights. Bake the crust until the sides are set, about 15 minutes. Carefully remove the weights and foil. Bake the crust until it is golden on the bottom, about 15 minutes longer. Cool the crust in the pan on a rack.
FOR THE LEMON CURD FILLING:
Bring the lemon juice to a boil in a heavy medium saucepan. Whisk the sugar and eggs in a medium bowl to blend. Gradually whisk in the hot lemon juice. Return the mixture to the saucepan.
Stir over low heat until the mixture thickens, about 5 minutes (do not boil). Remove from the heat.
Gradually whisk in the butter, then the lemon peel. Transfer the lemon curd to a medium bowl. Press plastic wrap directly onto the surface of the curd; refrigerate until cold.
TO ASSEMBLE AND SERVE:
Spoon the cold lemon curd into the crust at least 1 hour before serving, or up to 8 hours ahead. Cut the tart into wedges and serve.