Preheat oven to 375°F. Melt butter in a large skillet over medium. Add apples, brown sugar, lemon juice, cinnamon, and salt. Cook, stirring often, until apples are just tender, 12 to 14 minutes. Whisk together cornstarch and water in a small bowl. Stir cornstarch mixture into apples. Cook, stirring constantly, until mixture has thickened and apples are glazed, 1 to 2 minutes. Remove from heat, and cool completely, about 45 minutes.

Meanwhile, unroll piecrusts on a lightly floured surface. Using a 5-inch round cutter and a 4-inch round cutter, cut crusts into 12 (5-inch) circles and 12 (4-inch) circles, rerolling dough as needed. Place 4-inch circles in 1 layer on a parchment paper-lined baking sheet; chill 15 minutes.

While 4-inch dough circles chill, press 1 (5-inch) circle in bottom and up sides of each cup of a lightly greased (with cooking spray) 12-cup muffin pan, flattening any bunches of dough along sides.

Spoon filling evenly into each muffin cup (about ⅓ cup each). Cut 1 (4-inch) dough circle into 6 strips (just over ½ inch wide). Arrange strips in a lattice pattern over 1 muffin cup of filling. Trim excess dough, and press edges of lattice to seal. Repeat with remaining 4-inch dough circles and muffin cups. Freeze pan for 30 minutes.

Remove pan from freezer. Brush each piecrust lightly with egg, and sprinkle evenly with turbinado sugar.

Bake in preheated oven until crust is golden brown, 24 to 32 minutes. Transfer pan to a wire rack; cool 15 minutes. Using a small knife or offset spatula, carefully loosen pies; transfer to a wire rack. Serve warm, or cool completely.