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1
Whisk flour, sugar, and salt together in a bowl. Toss butter in the flour mixture until coated. Transfer mixture to a food processor and pulse in short bursts until butter is the size of hazelnuts. Drizzle ice water through the feed tube, pulsing in quick 4-second bursts until dough comes together in a ball.
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2
Divide dough into 2 equal balls. Flatten into disks and wrap each disk in parchment paper and plastic wrap. Refrigerate disks until firm, about 1 hour.
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3
Preheat the oven to 425 degrees F (220 degrees C).
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4
Roll out 1 disk into an 11-inch circle. Transfer it to a 9-inch pie plate. Trim the overhang to 1 inch.
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5
Bake in the preheated oven until golden, 15 to 20 minutes. Let crust cool while you make the filling. Keep oven on.
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6
Toss strawberries, blueberries, rhubarb, lemon zest, and vanilla extract in a large bowl. Mix sugar, cornstarch, and salt in a separate bowl. Toss fruit mixture in the sugar mixture just before pouring over the crust.
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7
Beat egg and water together in a small bowl to make egg wash. Brush the rim of the bottom crust with the egg wash.
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8
Roll out second disk into an 11-inch circle. Carefully position over the filled pie. Trim the edges and tuck the top crust over the rim of the bottom crust to form a tight seal. Crimp the edge decoratively. Cut an X-shape into the top crust. Brush the top with egg wash and sprinkle with raw sugar.
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9
Place pie on a baking sheet and bake for 20 minutes, turning halfway through. Reduce temperature to 350 degrees F (175 degrees C). Continue baking until crust is golden and the juices have thickened and start bubbling out through the X on the top, 35 to 40 minutes.
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10
Place pie on a rack to cool completely, at least 1 hour.
