Prepare the Dough: In a large mixing bowl, combine the pastry flour and a pinch of salt. Cut in chilled unsalted butter until the mixture resembles coarse crumbs. Stir in vinegar and cold water until a dough forms. Wrap tightly in plastic wrap and refrigerate for at least 1 hour.

Make the Filling: Combine chopped rhubarb and strawberries in a mixing bowl. Sprinkle sugar over the fruit and gently mix. Let the mixture sit for about 15-20 minutes to create a flavorful filling.

Roll Out Dough: Dust your countertop with flour. Divide the dough into two portions and roll one to about 1/8-inch thick.

Assemble Pies: Cut the dough into circles, place a tablespoon of filling on one half, fold over, and seal the edges.

Bake: Preheat your oven to 375°F (190°C). Brush the tops with egg wash and bake for 25-30 minutes until golden brown.

Cool and Serve: Allow to cool on a wire rack for about 10 minutes before serving warm or at room temperature.