Ingredients Adapted from Ina Garten
2 sheets ready rolled, all butter puff pastry
2 tablespoons Dijon mustard
12 thin (not paper thin) slices soppressa salami
150g Gruyere cheese, grated
1 egg beaten with 1 tablespoon water, for egg wash
Preheat the oven to 220C. Place a piece of parchment paper on a sheet pan. Place one piece of pastry on the sheet pan and brush the pastry with all the mustard, leaving a 1-inch border. Arrange the salami in overlapping layers on the mustard and sprinkle the grated cheese evenly over it, also avoiding the border. Brush the border with the egg wash. Lay the second piece of pastry directly on top of the first square, lining up the edges.
Brush the top with the egg wash, cut three large slits for steam to escape, and chill for 15 minutes. When the pastry is cold, trim the edges with a very sharp knife to make a clean edge. Bake the pastry in the center of the oven for 20 to 25 minutes, turning the pan around once, until puffed and brown. Allow to cool for a few minutes, cut in squares, and serve hot or warm.