Preheat the oven to 425 degrees. Line a baking sheet with a Silpat (a French nonstick baking mat). In a medium bowl, add strawberries, sugar, and cornstarch; stir to combine, set aside.
On a lightly floured work surface, roll dough into a large rectangle, about 1/8 inch thick. Using a 5-inch cookie cutter, cut out 6 rounds. Transfer rounds to prepared baking sheet.
Place about 2 tablespoons of the strawberry mixture onto one-half of each round. Lightly brush egg around the edge of the covered half of each round. Fold remaining dough over to enclose, forming a half moon. Gently press edges together to seal. Brush the tops of each pie with egg. Using a paring knife, slash the top of each pie. Sprinkle generously with sanding sugar.
Bake until golden brown, 20 to 25 minutes. Transfer pies to a wire rack to cool slightly before serving. Or let cool completely, and store in an airtight container at room temperature up to 4 days.