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To make the crust: Divide the pastry into two pieces — one about 2/3 of the total, the other 1/3. Roll the larger piece of pastry into a round, and place it in a 9" pie pan. Refrigerate for at least 30 minutes.
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Preheat the oven to 425°F.
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To make the strawberry-rhubarb filling: Whisk together the sugar, enhancer, and salt, and toss with the rhubarb and strawberries. Set aside.
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To make the blueberry filling: Whisk together the sugar and enhancer. Toss the blueberries with the sugar mixture, then stir in the lemon juice.
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Fill a 90° wedge of the pie shell with blueberry filling and the remaining 270° wedge with strawberry-rhubarb filling. See "tips," below, for suggestions how to do this.
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Roll the remaining crust into a 10" x 6" rectangle about 3/8" thick. Cut the dough, lengthwise, into five to six 3/4"-wide, 10"-long strips. Cut three to five 1 1/4" stars from the remaining dough.
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Place the strips parallel to one another over the strawberry-rhubarb section, cutting any excess off the ends and pinching them to the edge of the crust.
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Place the stars atop the blueberry filling.
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Brush stars, stripes, and outer crust with water, and sprinkle with sparkling sugar, if desired.
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Place the pie on a baking sheet (to catch the drips), and bake for 30 minutes. Reduce the oven heat to 375°F and bake for an additional 45 to 60 minutes, until the filling is bubbling and the crust nicely browned.
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Remove the pie from the oven, and let it cool for at least an hour before serving. If you can't wait, the pie may be served warm but it'll be a bit messy; it sets as it cools.