01 -
Set oven to 400°F (204°C) and line a large baking sheet with parchment paper.
02 -
Slice puff pastry sheets into 5-inch squares and refrigerate until ready to use.
03 -
In a medium saucepan over medium heat, melt unsalted butter. Add all-purpose flour and kosher salt; cook for 1 minute while stirring constantly.
04 -
Gradually pour whole milk into the saucepan, whisking continuously to avoid lumps. Continue whisking until the mixture is smooth and reaches a gentle boil.
05 -
Reduce heat to low and simmer, stirring occasionally, until the sauce begins to thicken.
06 -
Add fresh spinach to the sauce and cook for 1 minute, stirring, until wilted.
07 -
Stir in diced ham and 1 cup grated Swiss cheese. Mix thoroughly, remove pan from heat, and set filling aside.
08 -
Arrange puff pastry squares evenly on the prepared baking sheet. Add a generous spoonful of ham and cheese mixture to the center of each square and sprinkle tops with reserved grated Swiss cheese.
09 -
Fold the corners of each pastry square towards the center to form a compact parcel. Brush all exposed pastry with beaten egg white.
10 -
Bake parcels for 18–20 minutes or until the pastry has puffed and achieved a golden, crisp finish.