Preheat oven to 350°F/180°C in a fan forced oven. Prepare baking sheet with parchment paper.
Cut puff pastry sheet into 4 squares. Lay a slice of Swiss Cheese on the centre of the square. Place a tablespoon of cranberry sauce, turkey and onions on the cheese. Fold the puff pastry over to make a triangle. Using a fork, press down the edges to seal the turnover.
In a small bowl, beat together the egg yolk and milk to make the egg wash. Brush the egg mixture over each puff pastry turnover.