Preheat oven to 400ºF. Peel, core, and cut the apples into ¼” slices. Place in a bowl, squeeze lemon juice and salt over apples, and gently toss. Set aside. Carefully unfold both sheets of thawed puff pastry. Place dough on two baking sheets lined with parchment paper. Cut each sheet into 4 equal squares. Fold dough over about ⅛ of an inch around each square to form a thin border. Place slices of apple (overlapping) diagonally across the pastry. Add 2-3 slices on either side of the diagonal slices. Repeat until dough and apples are used up. Sprinkle each square with sugar. Dot each square with 1 tbsp of chilled butter. Bake for 30 minutes, or until pastry is golden-brown. Remove from oven and, while still warm, brush caramel sauce over apples. Can be frozen after cooking.