Add the lemon juice to a large bowl. Peel, core and cut the apples into 1/2-inch pieces, adding them to the bowl with the lemon juice as you go to prevent browning.
Melt the butter in large skillet over medium-high heat. Add the apples, granulated sugar, apple pie spice and salt; cook, stirring occasionally, until slightly softened, about 8 minutes. Sprinkle in the cornstarch and simmer until the juices thicken, 1 to 2 minutes.
Preheat the oven to 375 degrees F. Line two rimmed baking sheets with foil or parchment paper.
Unroll the pie crusts and cut out 16 rounds using a 3-inch cutter. Dollop 1 tablespoon of the apple filling into the center of half of the rounds. Cover each with a second round, seal with egg wash and crimp the edges with the tines of a fork. Place the pies on the prepared baking sheets and brush the tops with egg wash. With a sharp knife, cut a 1-inch vent in the top of each pie.
Bake until golden brown all over, 20 to 25 minutes. Let cool in the pan for 5 minutes, then transfer the pies to a rack to cool until warm or room temperature.
Before serving, put the confectioner's sugar in a small sieve and tap it with your hand to evenly dust the pies.