Make the crust: Combine the flour, sugar, and salt in a mixing bowl. Add the cubed butter and work into the flour using a pastry cutter or two butter knives until it resembles moist breadcrumbs. Add the ice water and stir together. Knead gently until a loose dough forms and wrap in plastic wrap, patting out into a disc. Chill for two hours in the fridge.
Once the dough is chilled, preheat the oven to 425 F. Combine the frozen berries, sugar, cornstarch, and lemon juice in a mixing bowl and toss.
Unwrap the dough and dust the counter with flour. Roll out the crust, turning often to keep from sticking, until slightly under ¼-inch thick. Use a glass or cookie-cutter about ½-inch wider than the muffin or tart opening to cut 8 circles. Use these to line the muffin or tart tin.
Fill each pastry-lined cup with fruit filling. Brush the edges with the egg wash. Cut 8 more circles and press them onto the tops of the mini pies. Use a fork to crimp the edges and trim the excess. Paint lightly with egg wash, sprinkle with sugar, and use a knife to cut tiny air vents into the top of each pie.
Bake for about 20-25 minutes, or until the pies have browned and the filling is bubbly. Remove and let cool for 1 minutes before remove the pies. Let cool on a rack until room temperature.
Serve atop a bucket of ice cream.
