Using a rolling pin, thin out the puff pastry sheets (sprinkle a little flour if needed to prevent from sticking).
Spread the mustard on one half of the puff pastry sheet. Layer with cheese and ham.
Fold the puff pastry over. Crimp the edges with a fork. Place it on a parchment paper=lined baking sheet. Brush the oil on top.
Bake for about 20-25 minutes until the puff pastry is golden brown. Once done, remove from the oven and set aside for 3-5 minutes. Slice in half to portion and serve.