Cut puff pastry sheets into 5-inch squares and refrigerate to keep them chilled. Preheat your oven to 400°F (200°C).

Melt butter in a medium saucepan over medium heat and whisk in flour and salt until golden, about 2 minutes. Gradually pour in warmed milk, stirring until smooth and simmering, about 7 minutes.

Reduce heat and let the béchamel simmer, stirring continuously for about 10 minutes until thickened.

Stir in chopped spinach into the béchamel, let cook for 1 minute. Fold in diced ham and half of the grated cheese.

Line a baking sheet with parchment paper. Place 1-2 tablespoons of the béchamel mixture in the center of each puff pastry square. Sprinkle remaining cheese over the filling and fold the corners of the pastry squares to close them.

Brush the tops with the beaten egg and bake in the preheated oven for 20 minutes or until puffed and golden brown.