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1
Prepare crust: On a lightly floured board, stack 2 pastry disks (1 pkg.; enough for a 2-crust pie) or use 1 batch Best Basic Pie Crust Dough and roll into a 16- by 21-in. rectangle, applying a little more pressure at edges to even out corners. Fit pastry into a well-buttered and floured 13- by 18-in. rimmed baking pan (aka half-sheet pan), filling corners; roll edges under on top of rim and crimp slightly so pastry sticks to pan rim. Chill, covered with plastic wrap.
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2
Reflour board, stack remaining pastry disks (or use remaining batch Best Basic Pie Crust Dough), and roll into a 12- by 17-in. rectangle. With a small star-shaped cookie-cutter, cut out 8 or 9 stars for the upper left quadrant of flag pie. For stripes portion of flag, use a ruler and a large knife to cut 7 or 8 (1/2-in.-wide) strips: 3 of them 9 1/2 in. long and the rest 15 1/2 in. long. Sprinkle a large flat baking sheet with flour, set pastry cutouts on baking sheet, and chill, covered with plastic wrap.
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3
Meanwhile, make filling: Put blueberries in a small bowl and toss with 3 tbsp. sugar, 2 tsp. cornstarch, and 2 tsp. lemon juice. In a large bowl, combine strawberries and raspberries and gently toss with remaining sugar, cornstarch, and lemon juice.
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4
Preheat oven to 375u00b0 with a rack in bottom position. Pour blueberry filling into top left quadrant of crust, for star area (about 5 by 6 in.). Pour strawberry-raspberry filling into rest of crust. Arrange cutout pastry stars over blueberries and stripes over strawberry-raspberry filling.
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5
With a fork, blend egg and cream together in a small bowl. Brush stars and stripes with egg wash and sprinkle with turbinado sugar.
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6
Bake pie on bottom rack until pastry is golden brown and filling is bubbling, 60 to 70 minutes; if the fruit is getting too dark, cover loosely with foil (check halfway through).
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7
Let cool on a rack for at least 2 hours before serving.
