Roll out a flaky pastry sheet and place 3 slices of ham on top to cover the entire sheet, leaving 2 cm of pastry free on the edge. Cover the ham with gruyère, then place 3 more slices of ham on top of the gruyère and add another layer of gruyère. Place the second pastry sheet on top and press the edges to stick the two sheets together. Mix the egg yolk with a tablespoon of water in a bowl, and with your fingers, apply the egg yolk mixture to the top of the pastry. Bake at 400°F for 30 to 45 minutes. It’s cooked when the top of the pastry is golden brown.