Make the filling: Place the butter in a large bowl and set aside. In a wide bowl sprinkle the gelatin evenly over the lemon juice (be sure the gelatin does not clump).
In a medium saucepan, stir together the orange juice and soda. Bring to a boil and cook until reduced by half, about 1 cup. Turn the heat to low and whisk to release extra heat.
In a medium bowl, whisk together the lemon and orange zest, eggs, egg yolks, and sugar. Pour into a saucepan and cook over medium low heat, whisking constantly, until a candy thermometer reads 180 degrees, or the curd can be easily coats the back of a wooden spoon.
Remove pan from heat and add the gelatin mixture. Whisk until gelatin is completely combined. Pour the liquid through a fine mesh sieve directly onto the butter. Whisk mixture furiously until it has increased in volume a bit. Cover the top of the curd with plastic wrap, pressing directly onto surface, and refrigerate for at least 4 hours.
Make the dough: In the bowl of a standing mixer fitted with a paddle attachment, beat the butter, sugar, zest, and salt until light and fluffy. Add the egg and beat just until incorporated. Scrape down sides of bowl, add flour all at once, and beat just until dough forms a ball. Remove dough and shape into a disk with your ahnds, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes.
Dust a work surface with flour. Use a rolling pin to roll dough into a 10-inch circle about 1/4 inch thick. The dough will be sticky so make sure to keep work surface floured and turn over dough as needed.
Gently guide the dough into a 9-inch tart pan with a removable bottom and press into place. Roll the rolling pin over the pan to trim off the excess. Place the tart pan in the freezer for 30 minutes.
Preheat oven to 375 degrees.
Line tart shell with foil and fill three-quarters full with pie weights or dried beans. Bake for 15 minutes, then remove foil and weights and bake for 10 minutes longer, or until lightly browned. Transfer the tart pan to a wire rack to cool.
Assemble the tart: In the bowl of a standing mixer fitted with whisk attachment., beat the curd on high for 5 minutes, then spoon into tart shell and level the filling. Refrigerate for 1 hour to set completely.
Make the topping: Pour the cream into a chilled metal bowl and beat with a chilled whisk for 1 minute. Sprinkle sugar and soda on top and continue whisking vigorously until soft peaks form. The whipped cream may also be made in a stand mixer fitted with a whisk attachment. To serve, gently push up on tart bottom to release from pan. Top tart with whipped cream.