How to Make It
Mix the flour, powdered sugar, and fleur de sel in a stand mixer with the paddle attachment until well combined. Add the butter. Mix until the mixture resembles a coarse meal. Add the beaten egg over the dough. Mix just until the dough begins to form.
Gather the dough into a ball; flatten into a disk. Wrap the dough in plastic wrap; chill for at least 30 minutes and up to 1 day.
Preheat the oven to 350 degrees Fahrenheit. Roll out the dough on a lightly floured surface to a 12 1/2-inch round (about 1/8 – 1/4 inch thick). Tip: you can place the disk between sheets of wax paper to roll it out more easily.
Transfer to a 10-inch 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 aluminum foil and fill with pie weights (or dry beans). 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.
Bring the lemon juice to a boil in a heavy medium saucepan.
Whisk the sugar and eggs in a medium bowl to blend. (If using a stand mixer, you can whip until the egg and sugar mixture becomes a ribbon consistency.)
Gradually whisk in the hot lemon juice to the egg and sugar mixture.
Return the mixture to the saucepan. Stir over low heat and gradually increase the temperature 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.
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.