for the crust:
1 cup unbleached all-purpose flour
1/2 cup whole wheat flour
1 1/2 tbsp. sugar
1/2 tsp. kosher salt
3/4 cup (1 1/2 sticks) chilled unsalted butter, cubed
1 large egg
1-2 tbsp. whole milk
for the filling:
1/2 cup sugar
1 large egg, beaten
In a large bowl, whisk together the flours, sugar, and salt. Add the butter and work it into the dry ingredients, using two knives, a pastry cutter, or your fingertips. Add the egg and stir with a fork until a dough forms. If the dough is too crumbly, add the milk, a tablespoon at a time. Knead a few times, then flatten into a disk. Wrap in plastic wrap and chill at least 1 hour (but up to several days is ok).
Preheat the oven to 400 F and line a baking sheet with parchment paper. In a small bowl, stir together the cornstarch and 3 tbsp water to dissolve. Remove the crust dough from the fridge to let it soften slightly. In a large saucepan, combine the rhubarb, raspberries, and sugar and cook over medium heat, stirring frequently, for about 5 minutes or until the rhubarb has released its juices and the sugar has dissolved. Add the cornstarch mixture and allow to come to a boil. Set aside.
On a well-floured surface, roll out the dough into a 14-inch round. Spoon the filling onto the center of the round, leaving a 2-inch border. Fold the outside dough over the filling, pleating every few inches to give you a nice, roughly octagonal shape. Carefully brush the exposed crust with the egg – you don’t want any to get into the filling. Sprinkle the crust generously with raw sugar. Bake until the crust is golden-brown and the filling is thick and bubbling, about 40-45 minutes. Let cool on the baking sheet. Cut into wedges and serve, preferably with spiced whipped cream.
Adapted from Bon Appetit