Prep the pan: Preheat the oven to 350°F. Line an extra-large baking sheet (or two smaller sheets) with parchment paper or foil and lightly mist with cooking spray.
Make the filling: Use an electric mixer on low speed to beat the cream cheese, softened butter, milk, and onion until smooth and fluffy, about 30 seconds. Fold in the turkey, salt, and pepper.
Roll the pastry: Unfold one sheet of puff pastry on a lightly floured surface and roll it into an 11-inch square. Cut it into 4 equal squares. Keep the second sheet in the fridge until you're ready for it.
Fill and seal: Spoon about ⅓ cup of filling onto the center of each square. Dip a fingertip in water and run it along the edges. Fold each square corner to corner into a triangle, then press the edges firmly with a fork to seal.
Repeat: Place the pockets on the prepared baking sheet. Repeat with the second sheet of pastry and the remaining filling.
Brush and bake: Brush the tops with melted butter. Bake for 25 to 30 minutes, until golden brown on top and bottom.
Serve: Let cool for 5 minutes, then serve with warm gravy for dipping or spooned over the top.