Make the filling:
In a saucepan over medium heat, combine strawberries, sugar, cornstarch, lemon juice, and vanilla. Cook for 6–8 minutes until the mixture thickens. Let cool completely.

Make the dough:
In a large bowl, mix flour, sugar, and salt. Cut in the cold butter using a pastry cutter or your hands until it resembles coarse crumbs. Add ice water a tablespoon at a time until the dough comes together. Divide into two disks, wrap in plastic, and chill for 30 minutes.

Assemble the pies:
Preheat oven to 375°F (190°C). Roll out the dough on a floured surface to about 1/8-inch thickness. Cut into 4–5 inch circles or rectangles. Place 1–2 tablespoons of strawberry filling in the center of half the pieces. Top with remaining dough pieces and press edges to seal with a fork.

Bake:
Place pies on a parchment-lined baking sheet. Whisk egg and water together for the egg wash and brush over the tops. Sprinkle with coarse sugar if using. Cut a small slit in the top of each pie to vent.

Bake for 20–25 minutes, or until golden brown. Cool on a wire rack before serving.