For the pastry, place the flour, sugar, and salt in the bowl of a food processor fitted with the steel blade. Pulse a few times to combine. Add the butter and toss carefully with your fingers to coat each cube of butter with the flour. Pulse 12 to 15 times, or until the butter is the size of peas. With the motor running, add the ice water all at once through the feed tube. Keep hitting the pulse button just until the dough comes together. Turn onto a well-floured board, cut in half, and form into two disks. Wrap and refrigerate for at least an hour. (Wrap the second dough well and freeze, if not using.)
For the filling, place 3 tablespoons of water in small bowl, whisk in the cornstarch, and set aside. In a large heavy-bottomed saucepan, combine the rhubarb, raspberries, granulated sugar, orange zest, and orange juice. Cook over medium heat for 5 to 6 minutes, until some of the juices are released. Stir in the cornstarch, bring to a boil, lower the heat and simmer for 2 minutes. Refrigerate for 30 minutes, until cool.
Preheat the oven to 425 degrees F. Line a sheet pan with parchment paper.
Roll the pastry to an 11- to 12-inch circle on a lightly floured surface and transfer to the prepared pan. Pile the raspberry rhubarb mixture onto the pastry, leaving a 1 1/2-inch border all around. Fold the border up over the filling, pleating, if necessary, and pressing lightly. Brush the pastry with egg wash, sprinkle the pastry with turbinado sugar, and bake for 30 to 35 minutes, until the pastry is browned and the filling is thickened. Cool for 30 minutes and serve warm or at room temperature.