Preheat oven to 400°F. Line a large baking sheet with parchment paper. Set aside.
Cut the puff pastry sheets into 5-inch (13cm) squares. Set aside in the refrigerator.
Melt butter over medium heat in a saucepan. Add flour and salt. Cook for 1 minute, stirring frequently.
Slowly pour in the milk, while whisking constantly, until smooth and boiling.
Reduce heat to low and allow the mixture to simmer, stirring occasionally, until it begins to thicken.
Add in spinach and cook for 1 minute, or until wilted.
Add ham and 1 cup of Swiss cheese and stir to combine. Remove from heat and set the mixture aside.
Arrange the puff pastry squares evenly on the prepared baking sheet.
Top with a heaping spoonful of ham and cheese mixture, and sprinkle with grated cheese.
Fold the corners up onto the top, forming a smaller square.
Brush the pastry with the egg wash, making sure to coat all sides.
Bake for 18-20 minutes, or until puffed and golden.