01 -
Preheat the oven to 400°F. Line a large baking sheet with parchment paper and set aside.
02 -
Cut the thawed puff pastry sheets into uniform 5-inch squares. Return them to the refrigerator to keep chilled.
03 -
Melt unsalted butter in a saucepan over medium heat. Add flour and kosher salt, stirring constantly for 1 minute to form a roux without browning.
04 -
Gradually whisk in room temperature whole milk in a slow, steady stream. Continue whisking until the sauce is smooth and reaches a gentle boil. Reduce heat to low and simmer, stirring occasionally, until thickened and able to coat the back of a spoon.
05 -
Add fresh spinach to the béchamel sauce and cook for approximately 1 minute until wilted, preserving its bright green color.
06 -
Stir diced ham and 1 cup of grated Swiss cheese into the sauce until fully combined and the cheese has melted. Remove from heat.
07 -
Place chilled pastry squares on the prepared baking sheet, spacing about 2 inches apart. Spoon a generous amount of filling into the center of each square and sprinkle with remaining Swiss cheese.
08 -
Fold each corner of the puff pastry squares toward the center, forming a smaller square with an open center. Press lightly to adhere but do not seal completely to allow for expansion.
09 -
Brush all exposed pastry surfaces gently with beaten egg white to create a glossy golden finish after baking.
10 -
Bake on the middle oven rack for 18 to 20 minutes until the pastries have puffed and turned a deep golden brown with crisp edges and bubbling filling.