To make spiced caramel sauce, melt butter in a small saucepan over medium heat. Add sugar. Bring to a gentle boil, whisking occasionally. Cook for 3 min. Stir in vanilla, cinnamon, and nutmeg until mixed. Remove from heat and let stand 1 min. Whisk in cream until sauce is smooth.
Roll out each half of a 397-g pkg puff pastry into a 10-in. square. Place on a parchment-lined baking sheet. Toss 2 peeled apples, sliced 1/4-in. thick, with 1 cup spiced caramel. Divide filling over pastry squares, leaving a 2-in. border. Fold border up over apples, overlapping slightly. Brush crust with milk. Bake at 400F until apples are tender and crust is golden, 25 to 30 min. Cover with foil if crust browns too quickly.