Pre-heat your oven to 400°F.
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.
Pour in the warm milk in 2 batches, whisking constantly until smooth and boiling.
Reduce heat to low and simmer, stirring occasionally until sauce starts to thicken.
Add in spinach and cook for 1 minute, just until wilted.
Add ham and 1 cup of Swiss cheese and stir well. Set aside.
Arrange the puff pastry squares on a parchment-lined baking sheet.
Top with a heaping spoonful of ham and cheese mixture, and sprinkle with grated cheese.
Fold the corners onto the top, forming a smaller square.
Brush the pastry with the egg wash, making sure to coat all sides.
Bake for 20 minutes, until puffed and golden.