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Position one oven rack at the bottom level and the second rack in the middle of the oven. Preheat the oven to 425°F.
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To prepare the crust: Roll the pie dough into a 12" to 13" circle on a piece of plastic wrap, a silicone rolling mat, or in a pie bag that's been heavily sprinkled with gluten-free flour or flour blend. Invert the crust into a lightly-greased 9" pie pan, pressing it into the pan. Refrigerate it while you make the filling.
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To make the filling: Whisk together the sugar, tapioca, and salt.
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Toss the rhubarb and strawberries with the sugar mixture. Spoon the fruit into the pan, filling it about 3/4 full and mounding the filling a bit in the center.
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Place dabs of the butter atop the filling, then return the pie to the refrigerator.
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Roll the remaining crust into an 11" circle.
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Invert the crust atop the filling and, using your fingers or the tines of a fork, crimp and/or pinch the edge of the top crust with the edge of the bottom crust to seal. Cut a few slits in the center, for steam to escape.
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Bake the pie on the bottom oven rack for 20 minutes. Reduce the oven temperature to 350°F, move the pie to the middle rack, and bake for 30 to 40 minutes, until the filling is bubbling and the crust nicely browned; it's helpful to have a pie shield or some foil at the ready in case the edge of the crust begins to brown too quickly.
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Remove the pie from the oven, and let it cool for an hour or so, to set.
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The pie may be served warm, but it'll be a bit messy; it sets as it cools. Top with vanilla ice cream or whipped cream, if desired.