Make the Crust:

1. Into the bowl of a food processor (you could also do this using a pastry cutter or your fingers), pulse together the butter and flour, until the largest chunks of butter are the size of peas or large breadcrumbs. Ad the Confectioner’s sugar, beaten egg, and salt. Pulse again. Slowly add enough water that, while still pulsing, a soft dough begins to form.
2. Place the soft dough on a well-covered flour surface and roll into a 12-inch circle. Transfer the dough circle onto a 9″ tart pan or into a Springform pan, allowing the dough to cover both the sides and bottom of the pan. If your dough extends beyond the sides, or higher than about an inch over the bottom, just fold it inward. This will fortify the sides. Place the crust into the refrigerator for at least 30 minutes. During this time, preheat oven to 400F (205C) or 375F/190C for Convection. You can use this 30 minutes to prepare the Frangipane and rhubarb for the filling.
3. After 30 minutes in the fridge, Crush a piece of parchment, and then fit it directly over the surface and sides of the dough. Fill the paper shell with pie weights or dry beans, pressing them into the corners.
4. Place the Crust into the oven and bake for 15 minutes. Remove from the oven and carefully take the pie weights and parchment off of the dough. Return the dough to the oven and continue to bake until the surface appears dry, about 5-6 minutes. Cool on a rack for 10 minutes. Lower the oven temperature to 375F (190C) or 350F (176C) for Convection.

Make the Filling:

1. Using either a hand- or a stand-mixer, cream together the butter and sugar until pale and fluffy, about 5 minutes at Medium-High. It should feel lighter when you move it with a spatula or spoon. Add the Almond Flour (or finely ground almonds), egg, orange zest, almond extract, and salt, and mix until incorporated. the filling should be fairly thick, sort of like thick oatmeal.
2. Spoon the filling into the slightly cooled crust. Smooth with an offset spatula or knife.
3. Cut the rhubarb into small, thin strips, or small pieces, whatever you like, and press them into the frangipane. Sprinkle some shaved almonds over the top, if desired. (Alternatively, spread them across a sheet pan and roast 6 mins at 350F/76C) and add after baking the tart.
4. Place the tart in the oven and bake 45-55 minutes, until the top is slightly puffy and golden, and almonds, if using, are toasted. A skewer inserted into the tart should come out clean. Cool completely before serving. Even better, bake the day before serving – it’s even better the second day!
5. Top with roasted almonds, confectioner’s sugar, or whipped topping.
6. This tart keeps well in the refrigerator up to 5 days. (See Note)