8 oz thinly sliced Jambon de Paris
1/4 cup, plus extra for servingMoutarde de Dijon
2 14-17-oz. packages puff pastry, thawed overnight in refrigerator
All purpose flour (for surface)
4 oz. Swiss cheese, thinly sliced
1 large egg
1 tsp. heavy cream
Roll out 1 package of puff pastry on a floured surface to a 16x12" rectangle. Transfer to a parchment-lined rimmed baking sheet and chill while repeating the operation for the 2nd sheet of pastry. Let chill both sheets of pastry for 15 minutes.
Spread 1/4 cup Moutarde de Dijon evenly over 1 sheet of pastry, leaving a 1/2" border around the edges. Add a layer of ham and another one of cheese on top. Arrange the second pastry sheet on top, fold the borders and crimp edges with a fork. Let it chill for at least 30 minutes.
Preheat oven to 400°. Beat egg and cream in a bowl. Brush the mixture over the pastry. Cut thin slits in a decorative pattern into top of pastry to allow steam to escape. Bake the feuilleté for 15 minutes, rotate the pan and bake it another 15 minutes. The pastry has to be golden brown.
Let cool before to slice 1 1/2" squares. Serve with Moutarde de Dijon alongside.