This pie can be made with many different fruit combinations according to seasonality and personal preferences. Try it with peaches, apricots, plums, cherries, and berries in any combination you like.
Per serving:
Mix together ice water, vinegar, and egg yolk; set aside.
In bowl of food processor with steel blade, pulse all dry ingredients until well incorporated. Add butter and pulse until it has formed small pea-sized lumps, from 5 to 10 seconds.
With motor running, add liquid in slow stream. Pulse until dough is holding together, but not yet forming a ball. Remove dough from food processor bowl and form into a ball. Wrap in plastic and flatten to create disc. Refrigerate for at least 1 hour. Take out of refrigerator about 10 minutes before using.
Preheat oven to 425 F (220 C) and flour rimmed baking sheet with gluten-free flour.
Place large piece of parchment paper on work surface and place dough in middle. Cover with second piece of paper and roll out dough to form a 12 in (30 cm) circle. Remove top sheet and flip dough onto baking sheet. Remove paper.
Mix together fruits, hazelnut flour, maple syrup, and cornstarch. Place fruit mixture in centre of dough, up to 2 in (5 cm) from sides. Garnish with hazelnuts. Fold dough over, leaving centre uncovered. Whisk egg with 1 tsp (5 mL) water to create egg wash, and brush over pastry.
Bake for 15 minutes, then lower temperature to 375 F (190 C) and bake for an additional 25 to 30 minutes, until fruits are tender and dough is golden.