1. To make the Dough: Put the flour, confectioner's sugar and salt in a food processor and pulse a couple of times to combine. Scatter the pieces of butter over the dry ingredients and pulse until the butter is coarsely cut in - you should have some pieces the size of oatmeal flakes and some the size of peas. Stir the yolk, just to break it up, and add it a little at a time, pulsing after each addition. When the egg is in, process in long pulses - about 10 seconds each - until the dough, which will look granular soon after the egg is added, forms clumps and curds. Just before you reach this stage, the sound of the machine working the dough will change - heads up. Turn the dough out onto a work surface and, very lightly and sparingly, knead the dough just to incorporate any dry ingredients that might have escaped mixing.
2. To Press the Dough into the Pan: Butter a 9-inch fluted tart pan with a removable bottom. Press the dough evenly over the bottom and up the sides of the pan, using all but one little piece of dough, which you should save in the refrigerator to patch any cracks after the crust is baked. Don't be too heavy-handed - press the crust in so that the edges of the pieces cling to one another, but not so hard that the crust loses its crumbly texture. Freeze the crust for at least 30 minutes, preferably longer, before baking.
3. To Partially or Fully Bake the Crust: Center a rack in the oven and preheat the oven to 375 degrees F.
4. Butter the shiny side of a piece of aluminum foil and fit the foil, butter side down, tightly against the crust. (Since you froze the crust, you can bake it without weights.) Put the tart pan on a baking sheet and bake the crust for 25 minutes. Carefully remove the foil. If the crust has puffed, press it down gently with the back of a spoon. For a partially baked crust, patch the crust if necessary, then transfer the crust to a cooling rack (keep it in its pan).
5. To Fully Bake the Crust: Bake for another 8 minutes or so, or until it is firm and golden brown. (I dislike lightly baked crusts, so I often keep the crust in the oven just a little longer. If you do that, just make sure to keep a close eye on the crust's progress - it can go from golden to way too dark in a flash.) Transfer the tart pan to a rack and cool the crust to room temperature before filling.
6. To Patch a Partially or Fully Baked Crust, if Necessary: If there are any cracks in the baked crust, patch them with some of the reserved raw dough as soon as you remove the foil. Slice of a thin piece of the dough, place it over the crack, moisten the edges and very gently smooth the edges into the baked crust. If the tart will not be baked again with its filling, baking for another 2 minutes or so, just to take the rawness off the patch.
7. To make the Orange Filling:Have an instant-read thermometer, a strainer and a blender (first choice) or food processor at hand. Bring a few inches of water to a simmer in a saucepan.
8. Put the sugar and orange and lemon zest in a large heatproof bowl that can b e set over the pan of simmering water. Off the heat, rub the sugar and zests together between your fingertips until the sugar is moist, grainy and very aromatic. Whisk in the eggs, followed by the orange and lemon juice.
9. Set the bowl over the pan and start stirring with the whisk as soon as the mixture feels tepid to the touch. You want to cook the cream until it reaches 180 degrees F. As you whisk - you must whisk constantly to keep the eggs from scrambling - you'll see that the cream will start out light and foamy, then the bubbles will get bigger and then, as it gets closer to 180 degrees F, it will start to thicken and the whisk will leave tracks. Heads up at this point - the tracks mean the cream is almost ready. Don't stop whisking or checking the temperature, and have patience - depending on how much heat you're giving the cream, getting to temp can take as long as 10 minutes.
10. As soon as it reaches 180 degrees F, remove the cream from the heat and strain it into the container of the blender (or food processor); discard the zest.
11. Soften the gelatin in the cold water, then dissolve it by heating it for 15 seconds in a microwave oven (or do this in a saucepan over extremely low heat). Add the gelatin to the filling and pulse once just to blend, then let the filling cool to 140 degrees F, about 10 minutes.
12. Turn the blender to high (or turn on the processor) and, with the machine going, add the butter about 5 pieces at a time. Scrape down the sides of the container as needed as you incorporate the butter. Once the butter is in, keep the machine going - to get the perfect light, airy texture, you must continue to blend the cream for another 3 minutes. If your machine protests and gets a bit too hot, work in 1-minute intervals, giving the machine a little rest between beats.
13. Pour the cream into a container, press a piece of plastic wrap against the surface and refrigerate for at least 4 hours. (The cream can be refrigerated, tightly covered, for up to 5 days or frozen for up to 2 months; thaw overnight in the fridge.)
14. When you are ready to construct the tart, whisk the cream vigorously to loosen it. Spread the cream evenly in the crust. Arrange the orange segments in the center of the tart and prepare the glaze: bring the jelly and water to a boil. Use a pastry brush or pastry feather to lightly spread the jelly over the orange segments and cream. Serve now or refrigerate the tart until needed.