1 Make the candied citrus zest: Place lemon zest in a saucepan and cover with water by 1/2 inch. Bring to a boil. Remove from heat, drain the water. Fill up with water again and repeat. Bring to a boil, remove from heat, then drain.
Add water again, this time adding the sugar as well. Simmer for 30 minutes, drain.
2 Dry the candied zest: Spread the zest out on a sheet of parchment paper. Let dry overnight at room temperature (do not put in the refrigerator). (Tip: if you wrap the zest strands around a chopstick and let them dry wrapped, then the next day when you unwrap them, they'll hold a curly form.)
The next day toss the zest with a little more sugar.
3 Make the tart dough: Whisk together the flour, salt, and powdered sugar in a bowl. Add the butter and either use your (clean) fingers, or use a pastry blender, to mix the butter into the flour until the dough forms flaky crumbs and lumps. Try to make sure that all of the flour is coated with some butter.
With a wooden spoon, mix in the egg, 1/4 teaspoon almond extract, and 1 teaspoon of lemon zest. Continue to mix until the dough clumps.
4 Chill the tart crust: Shape into a ball, flatten into a disk, wrap in plastic, refrigerate at least an hour or up to 2 days in advance.
5 Pre-heat oven to 350°F.
6 Roll out the tart crust: If the dough has chilled overnight, let it sit at temperature for at least 10-15 minutes before attempting to work with it. This allows the dough to relax enough to become somewhat pliable before rolling out.
Roll out the dough by either sprinkling the dough and rolling surface with little flour, or you can roll the dough out between two sheets of parchment paper. Roll the dough out to a circle 2 inches wider in diameter than your tart pan (about a 12-inch round).
Press the dough into the tart pan. Use your rolling pin to level the dough along the edges of the pan. Freeze for 30 minutes.
7 Pre-bake the tart crust: Place aluminum foil over the foil and gently mold it to the dough in the pan. Add pie weights (dried beans, sugar, rice, or ceramic pie weights).
Bake at 350°F for 20 minutes. Remove from oven, lift the sides of the aluminum foil to remove the pie weights, and let the crust cool.
7 Make the lemon curd filling: Melt butter and crème fraîche together over a double boiler, stirring to combine. (If you don't have a double boiler, bring a couple inches of water to simmer in a saucepan, place a stainless steel or pyrex bowl on top of the saucepan, making sure the bottom of the bowl is not touching the simmering water.) Remove top pan from heat and set aside.
In a second bowl over the double boiler, whisk the eggs and egg yolks just long enough to warm them. Remove from heat and beat in the butter mixture, then the lemon juice.
Strain the mixture through a sieve into a clean bowl and place over the simmering water. Add the sugar and zest and whisk until warm to touch, about 4 minutes.
8 Assemble and bake the tart Pour the lemon filling into the crust and bake at 350°F for 25 minutes or until set in the middle.
9 Chill before serving: Let cool on a rack. Chill in the refrigerator. When ready to serve, top the tart with candied lemon zest.