Preheat the oven to 400 degrees F. Line two baking sheets with parchment paper.
In a medium bowl, whisk together the flour, sugar and salt. Using a pastry cutter or fork, cut the cold butter into the flour mixture, forming pea-size crumbles. In a separate bowl, whisk together the milk and 1 egg and add to flour mixture; work the dough until smooth, but with visible flecks of butter. Flatten the dough into a disc and wrap in plastic wrap; chill at least 1 hour before using.
Meanwhile, in a small saucepan, melt 1 tablespoon butter over medium heat. Add the strawberries and rhubarb and cook until soft and jam-like, about 6–8 minutes. Stir in the sugar, balsamic vinegar, 1 teaspoon lemon zest, ¼ teaspoon lemon juice and salt. Cook another 3 minutes. Let cool to room temperature, then cover and chill at least 30 minutes before using.
When the dough is properly chilled, roll the dough on a lightly floured surface into a ¼-inch thick (10x12 inch) rectangle. Cut the dough into eight (3x4 inch) rectangles. Gather the dough scraps, re-roll, and cut two more (3x4 inch) rectangles. Place 1 ½ tablespoons filling in the center of each rectangle. Whisk the remaining egg in a bowl and brush the edges of rectangles; fold one short side of the dough over to the other short side, encasing the filling. Crimp edges using a fork.
Place the pies on the parchment paper-lined baking sheets and chill 30 minutes, at least.
Once chilled, brush each hand pie with remaining beaten egg, poke each pie with a fork (so steam can release) and bake until pies are golden, about 20-25 minutes; let cool slightly before serving.
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