Heat the oven to 400° F and line the baking sheet with Heavy Duty Foil to keep the sauce from spilling out and burning onto the pan.
Roll out the puff pastry dough into a square and cut out 4 smaller squares. Set aside on the foil-lined baking sheet in the fridge.
In a pot over medium heat, add the butter and cook until melted.
Stir in the garlic and cook until tender, about 1 minute.
Using a wooden spoon, stir in the flour, ground mustard and nutmeg. Continue to stir for about a minute then slowly add the milk and bring to a simmer for 3 minutes or until thickened.
Stir in the cheeses, meat and spinach and salt to taste. Turn off the burner and set aside.
Pull out the puff pastry from the fridge. In a small bowl, whisk together the egg and water.
Place a large spoonful of filling in the middle of each pastry dough and fold in each corner like a packet.
Brush the corner pieces with the egg wash then place in the oven for 15-20 minutes or until golden.
Remove from the oven and eat hot or at room temperature.