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1
In a food processor, pulse the 2 1/2 cups of flour with the sugar and salt.
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2
Add the butter and sour cream and pulse until the mixture resembles small peas.
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3
Sprinkle on the ice water and pulse until the pastry just comes together.
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4
Transfer the pastry to a sheet of plastic wrap and pat it into a disk.
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5
Wrap up the pastry and refrigerate for at least 30 minutes or overnight.
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6
Preheat the oven to 400.
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7
In a large bowl, gently toss the raspberries and blueberries with the granulated sugar, cornstarch and lemon juice until well combined.
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8
Line a rimmed baking sheet with parchment paper.
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9
On a lightly floured work surface, roll out the pastry to a 15-inch round, about 1/8 inch thick.
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10
Transfer the pastry to the prepared baking sheet.
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11
Spread the berry filling on the pastry, leaving a 1 1/2-inch border.
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12
Fold the pastry border up and over the berries, pinching together any cracks.
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13
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.
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14
Let cool on the baking sheet for 20 minutes.
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15
Dust the cooled tart with confectioners' sugar.
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16
Carefully remove the tart from the baking sheet; it may stick.
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17
Transfer to a plate, cut into wedges and serve.
