How to Make It
In a food processor, pulse the 2 1/2 cups of flour with the sugar and salt. Add the butter and sour cream and pulse until the mixture resembles small peas. Sprinkle on the ice water and pulse until the pastry just comes together. Transfer the pastry to a sheet of plastic wrap and pat it into a disk. Wrap up the pastry and refrigerate for at least 30 minutes or overnight.
Preheat the oven to 400°. In a large bowl, gently toss the raspberries and blueberries with the granulated sugar, cornstarch and lemon juice until well combined.
Line a rimmed baking sheet with parchment paper. On a lightly floured work surface, roll out the pastry to a 15-inch round, about 1/8 inch thick. Transfer the pastry to the prepared baking sheet. Spread the berry filling on the pastry, leaving a 1 1/2-inch border. Fold the pastry border up and over the berries, pinching together any cracks. Brush the pastry border with milk and bake the tart for 45 to 50 minutes, or until the pastry is golden and the fruit is bubbling. Let cool on the baking sheet for 20 minutes. Dust the cooled tart with confectioners' sugar. Carefully remove the tart from the baking sheet; it may stick. Transfer to a plate, cut into wedges and serve.
Variation To make 12 individual tarts, increase the pastry recipe by half. Divide the pastry into 12 equal portions and pat into disks. Refrigerate for at least 30 minutes or overnight. Roll out into 7-inch rounds, about 1/8 inch thick. Spread 1/2 cup of the berry filling on each round, leaving a 1-inch border, then proceed through the rest of Step 3.