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1
In the bowl of a food processor, combine 1/2 cup ground hazelnuts, 2 cups of flour, salt, and sugar, and pulse until mixed. Add in the cold butter and pulse 20 times.
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2
In a separate, smaller bowl, combine the cold water, apple cider vinegar, and ice.
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3
Add this cold water mixture to the dough a couple of tablespoons at a time, pulsing until combined after each addition. [Editors' note: We used about 4 tablespoons of liquid total.]
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4
Once the dough in uniform, divide into two equal portions and wrap each in plastic. Chill dough for at least 3 hours, or even up to a month. When you are ready to use the dough, keep it in the fridge until you are ready to roll it out.
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5
In a small saucepan, melt the butter into the berries over low heat. When the butter is completely melted, add the honey and the lemon juice, and let the saucepan sit over low heat for about 15 minutes, stirring ever few. Add in the lemon zest and remove from heat. Let cool for 30 minutes, and prepare your bottom crust as you wait.
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6
Remove one of the dough halves from the fridge and carefully roll it out on a floured countertop until it's about 12 to 13 inches in diameter. (Because of the hazelnuts in the dough, the fat content is even higher than usual, and therefore the dough gets very soft, very quickly. If you find that your dough is getting too soft, re-wrap it in plastic, chill it for another hour, and try again.) After you have rolled the dough out for your bottom pie shell, carefully move this disc into your 9-inch pie pan. Smooth it into the pan and trim it around the top edges.
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7
Preheat the oven to 375u00b0 F. When the berry mixture is cool, stir in the cornstarch so that it's completely dissolved, and then pour the mixture into the prepared pie shell.
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8
For the top crust of the pie, combine 1/2 cup of the remaining prepared dough from the fridge with the 1/2 cup old-fashioned oats, the remaining 1/2 cup of the toasted, ground hazelnuts, and the remaining 2 tablespoons of flour.
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9
Pulse in a food processor until combined and the consistency of green peas. Carefully pour the oat-dough mixture over the top of the pie in a large circle, adding a layer of the mixture around the outer layer of the pan.
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10
Bake the pie for 40 to 45 minutes, and then let it cool completely, about 2 hours. Slice and enjoy with ice cream or by itself.
