Yield: 10 to 12 batons
Unfold the sheet of puff pastry on a well-floured board, and roll it to an 11-by-13-inch rectangle with a floured rolling pin. (Diagonal strokes keep the pastry rectangular.) With a shorter end closest to you, brush the lower half of the pastry evenly with the mustard, leaving a 1/2-inch border around the edges. Brush the border of the pastry with the egg wash and fold the top half over the bottom half, lining up the edges. Place the pastry on a sheet pan lined with parchment paper and chill for 15 minutes.
Place the pastry on a board and trim the three irregular edges with a sharp knife. With the folded edge away from you, cut the pastry in 1-by-6-inch strips. You will have 10 to 12 batons. Spread the batons out on the sheet pan so they’re not touching. Brush the tops lightly with the egg wash (don’t allow the egg wash to drip down the sides) and sprinkle evenly with the Gruyere, Parmesan, and 1 1/2 teaspoons sea salt. Chill for at least 15 minutes.
When ready to bake, preheat the oven to 400ºF.
Bake the batons for 15 to 18 minutes, until golden brown and puffed. Allow to cool on the pan for 3 minutes and serve warm.