Preheat the oven to 400°F. Prepare a long nonstick tart pan, around 14x4.5-inch, preferably with fluted edges. Grease it with 1 teaspoon canola oil.
In the bowl of a stand mixer fitted with a paddle attachment, combine the flour with salt, ½ cup powdered sugar, and the cold butter, until fine crumbs form. Add egg yolks and knead the dough with your hands until perfectly smooth. Tightly wrap in plastic and refrigerate for at least 1 hour.
Roll out the dough and place it into the tart pan. Thoroughly line the bottom and sides of the pan, then trim excess dough edges. Line the crust with foil and fill with pie weights or dried beans. Bake for 15 minutes. Remove pie weights and bake for 20 minutes more, until the crust is golden. Remove from the oven and cool in the pan on a wire rack.
Place gelatin and 2 tablespoons cream in a small bowl and let stand until gelatin is softened, 5-10 minutes. Warm remaining cream in a small saucepan over medium heat until steaming, about 3 minutes. Remove from heat, add gelatin mixture, and whisk until smooth. Whisk in mascarpone, the remaining powdered sugar, and citrus zest and juice. Pour into the pie crust. Chill overnight, allowing to set. Just before serving, garnish with slices of lemon and lime, meringues, fresh mint, and powdered sugar.