Ham and Cheese Puff Pastry

1.Preheat the oven to 450. Line a baking sheet with parchment paper. On a floured surface, roll out the dough to a 10" square. Transfer pastry to baking sheet.
2.Brush mustard over all of the dough leaving a ¾" border. Place the ham slices, slightly overlapping each other on the bottom half of the dough, again leaving a border for crimping. Cover the ham with the cheese.
3.Fold the top half of the dough over the ham and cheese, and press the edges closed gently with your fingers. Use a fork to crimp the edges. Brush the egg yolk over the entire exposed surface liberally.
4.Bake for 17-19 minutes, until the pastry is golden brown. Let cool for a few minutes before slicing an serving.