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1
Preheat oven to 350 degrees F (175 degrees C).
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2
Sift 2 cups gluten-free flour, 2 tablespoons sugar, and 1/4 teaspoon salt together in a bowl 3 times. Add margarine; rub it into the flour, lifting and stirring with your fingers, until mixture resembles fine breadcrumbs. Make a well in the center of the mixture.
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3
Whisk 1 egg white and 2 1/2 tablespoons water together in a bowl; pour into the well. Mix with a butter knife using cutting motion until dough holds together. Add water, 1 teaspoon at a time, if dough is too dry. Add flour, 1 teaspoon at a time, if dough is too sticky.
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4
Turn the dough out onto a lightly floured surface. Gently press into a ball, cover in plastic wrap, and refrigerate for 30 minutes.
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5
Grease a 9-inch pie plate heavily with margarine. Preheat oven to 375 degrees F (190 degrees C).
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6
Remove dough from the refrigerator. Roll out between 2 pieces of parchment paper using a rolling pin until approximately 10 inches in diameter. Place the crust into the pie plate, trim the edges, and use a fork to decorate the rim. Prick the base of the crust with a fork. Line with a double layer of aluminum foil and a layer of pie weights or dried beans.
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7
Bake in the preheated oven until edge of crust is golden, about 10 minutes. Carefully remove the foil and weights; continue baking until the crust is starting to turn golden, about 15 minutes more. Remove from oven.
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8
Sift 1 1/4 cups white sugar, 1/3 cup gluten-free flour, and 1/3 cup gluten-free cornstarch together into a saucepan. Whisk in lemon juice slowly until smoothly combined. Turn heat to medium and gradually whisk in 1 cup water. Continue whisking until the mixture begins to thicken, 3 to 5 minutes. Remove from heat; whisk in 6 egg yolks and 2 tablespoons margarine. Return to low heat and stir constantly until very thick, about 2 minutes. Spread filling over the crust. Allow to cool, about 5 minutes.
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9
Beat 7 egg whites together in a clean, dry bowl using an electric mixer until soft peaks form. Add cream of tartar and beat until peaks stiffen. Add 1 3/4 cups plus 2 tablespoons sugar in batches, beating on high after each addition, until meringue is thick and glossy. Beat in the vanilla extract. Spoon meringue gently over the lemon filling, making peaks with a knife.
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10
Bake in the still-hot oven on the center rack until meringue begins to turn golden, 10 to 15 minutes. Turn the oven off; allow pie to cool in the oven 8 hours to overnight.
