Step 11 to 2 days before making the tart, make the pastry. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, orange zest and sea salt at low speed for 1 minute. Scrape down the sides of the bowl and the beater. Add the powdered sugar and mix together at low speed. Stop the machine and scrape down the sides of the bowl and beater again, then add the almond flour and vanilla extract and beat at low speed until combined. Add the egg and 1/4 of the flour and beat at low speed just until incorporated. Gradually add the remaining flour and beat just until incorporated.
Step 2Divide the dough in half. Pat each piece into a 1/2 inch thick disk and wrap airtight with plastic. Refrigerate one for at least one hour and preferably several hours (the second can be frozen to save for another use).
Step 3Roll out the pastry to a 1/4-inch thick circle, about 11 inches in diameter and carefully line a 9-inch tart pan. Lightly pierce the bottom a few times with a fork, taking care not to go all the way through, and place in the refrigerator, uncovered, until dry and no longer pliable, 1 to 2 hours. Double wrap with plastic and freeze overnight.
Step 4Pre-bake the crust: Heat the oven to 325 degrees. Unwrap the tart shell and place the pan on a baking sheet. Bake until the shell is golden brown, with no trace of moisture, about 30 minutes, rotating the pan halfway through for even coloring. Remove from heat and allow to cool completely.
Step 1Slice the ends off the oranges and then, using a sharp knife, cut them into thin slices (about 1/8 inch thick). Transfer to a bowl as you slice them and remove any visible seeds.
Step 2Pour off the juice from the bowl with the orange slices into a measuring cup. Add enough freshly squeezed juice to measure 2 1/2 cups. In a saucepan, combine the juice with the ½ cup sugar and orange zest. Bring to a boil over medium heat, stirring until the sugar has dissolved.
Step 3Add the sliced oranges to the saucepan and press down to be sure they are all submerged. Reduce the heat to low, cover partially and simmer 15 minutes. Remove from the heat and set aside until the orange have cooled in the syrup.
Step 4Place a strainer over a bowl and carefully drain the oranges. Pat the slices dry with paper towels. Some of them may have split in the middle, which is fine. If desired, cut the slices in half. Save the poaching liquid for another use (it makes a delicious sorbet).
Step 5Heat the oven to 375 degrees. Place the fully baked tart shell on a sheet pan. Spread the orange-rhubarb compote in an even layer over the bottom of the pastry. Arrange the orange slices, slightly overlapping, in concentric circles in the pastry shell. Place the sheet pan with the tart in the center of the oven and bake until the oranges are soft and some of the edges are beginning to brown, 30 to 35 minutes.
Step 6To finish the tart, sprinkle the vanilla sugar or additional sugar over the top. Using a kitchen blow torch, heat the sugar until it melts and glazes the oranges or caramelizes lightly and begins to brown in some spots. You can do this under the broiler but you will have to watch very closely so you don’t burn the pastry shell. Cool to room temperature before serving.