1 large egg, beaten
1/4 cup whole grain mustard
1/2 pound thinly sliced Black Forest Ham
1/4 to 1/2 pound Gruyere cheese, sliced (depending on how much cheese you like)
Place a rack in the center of the oven and preheat oven to 400 degrees F. Line a baking sheet, or round pizza pan with parchment paper. Set aside.
Unfold or unroll one sheet of puff pastry on a lightly floured surface. Gently roll with a rolling pin until each side is increased by about 1/2-inch. This is just a light rolling. Place the pastry on the prepared baking sheet.
Brush pastry, from edge to edge with egg wash. Spread mustard atop the egg wash, leaving 1/2-inch of space between the mustard and the edge of pastry. Layer on ham slices. Top with sliced cheese.
Gently roll out the second piece of puff pastry. Layer on top of the ham and cheese pastry. Press the edges together with your fingers. Gently fold about 1/2-inch of the edges over, creating and thick crust. Press with the tines of a fork. Dip the fork in flour if the fork sticks to the puff pastry during pressing.
Brush with egg wash from edge to edge. Use a sharp knife to cut three venting holes through the top pastry.
Bake for 20 to 25 minutes or until puffed and golden brown. Allow to cool for 10 minutes before serving. Serve warm.