In a large mixing bowl, combine all-purpose flour, sugar, and salt. Cut in the cold butter until mixture resembles coarse crumbs. Gradually add cold water until dough holds together.

Wrap the dough in plastic wrap and flatten into a disc. Refrigerate for at least 30 minutes.

Mix together blueberries, sugar, lemon zest, and cornstarch. Toss gently until blueberries are evenly coated, then set aside.

Roll out the dough on a lightly floured surface to 1/8 inch thickness. Cut rounds for muffin tin cups.

Place the rounds into a greased muffin tin, fill with blueberry mixture. Cut additional rounds for lids and brush with beaten egg.

Bake in a preheated oven at 400°F (200°C) for 25 to 30 minutes until golden brown and bubbly.

Let cool in the muffin tin for 10 minutes, then transfer to a wire rack to cool completely before serving.