Preheat the oven to 375°F (190°C). Line a muffin tin with a light coating of nonstick spray.
Prepare the filling:
In a medium bowl, mix the diced apples, granulated sugar, brown sugar, cinnamon, nutmeg, lemon juice, flour, vanilla extract, and salt until the apples are evenly coated.
Prepare the crusts:
Roll out the pie dough on a lightly floured surface. Use a round cookie cutter (about 4 inches wide) to cut out circles. Press each circle into the muffin tin, making sure the dough comes up the sides a little to create a mini pie crust.
Fill the pies:
Spoon about 1-2 tablespoons of the apple mixture into each crust, filling them generously but not overflowing.
Top the pies:
You can either cut smaller rounds to place on top, create a lattice with strips of dough, or leave them open-faced. If covering, make sure to cut small slits for steam to escape.
Egg wash:
In a small bowl, whisk together the egg and milk (or water). Brush the tops of the pies with the egg wash for a shiny, golden finish. Sprinkle a little extra sugar over the top if desired.
Bake:
Place the muffin tin in the oven and bake for 18-22 minutes, or until the crusts are golden brown and the filling is bubbly.
Cool:
Allow the mini pies to cool in the tin for about 10 minutes, then carefully remove them and let them cool completely on a wire rack.