Open the sealed puff pastry package. It will be folded in thirds. With a sharp pointed knife, cut along 1 fold to separate 1/3 of the dough. Cut that third into 2 equal pieces. Seal and refreeze the remaining dough.
Preheat the oven to 450°F.
While the oven is heating, pare, quarter, core, and thinly slice the apple. Sprinkle the lemon zest and lemon juice on the apple.
When the oven is hot, lightly roll the dough pieces to form two 4 1/2-inch squares. Place the dough pieces on a nonstick cookie sheet that has been coated with cooking spray. Lightly coat the dough with cooking spray. Top each half of the dough with the applesauce, sugar, nutmeg, and apple slices, leaving a 1/2 inch border. Coat once again with cooking spray.
Bake for 15 minutes. Check after 10 minutes and lower the heat to 425°F. Bake until the crust is golden and some of the apple edges have browned. Serve warm.