Preheat your oven to 400°F (200°C).
Roll out the puff pastry and cut into squares (3x3 inches).
Place a slice of corned beef in the center of each square.
Add a slice of Swiss cheese and a spoonful of sauerkraut on top of the beef.
Drizzle a small amount of thousand island dressing on top.
Fold the pastry over the filling and seal the edges.
Brush the tops with beaten egg.
Arrange on a baking sheet lined with parchment paper.
Bake for 15-20 minutes or until golden brown.
Let them cool slightly before serving.
