Melt butter in a saucepan, whisk in flour and cook for a few minutes to remove the raw flour taste, it should foam up a bit. Whisk in milk and continue stirring until it thickens and comes to a boil. Stir in nutmeg, mustard and salt to taste. Remove from heat to cool.
Lightly roll out puff pastry, just to make an even thickness. Place on a cookie sheet and brush edges with milk. Spoon béchamel onto the pastry, leaving a ½ inch border. Top with ham and then grated cheese. Bake until golden brown, puffy and crisp on the bottom, begin checking after 25 minutes. Allow to rest for a few minutes before cutting into squares, it will deflate so don't be alarmed. This is great served with a arugula salad to balance the richness.