Preheat oven to 400 with rack in center position. Lightly flour a work surface and rolling pin. Roll out puff pastry to a 10 x 15 inch rectangle, about ¼” thick. Place on a sheet of parchment paper so that one of the short sides is facing you. Use a sharp knife to cut off the top corners of the rectangle at a 45 degree angle, leaving 4 inches of dough between them. Next cut out 2 triangular notches at the bottom of the rectangle, leaving 4 inches between them as well. It should look like a short stumped christmas tree missing its pointing top. Finally make 8 diagonal slits, about 1 inch apart and parallel to the notches, along each side of the pastry, leaving a 4 inch wide strips in the center intact.
Spread Dijon over the center strip of pastry (not flaps), and layer the cheese and ham over the mustard leaving a ¾” border on top and bottom. Fold bottom and top edges over the ham and cheese. Starting from top and alternating sides, fold the diagonal flaps of pastry over the filling, crisscrossing them over one another, until all strips are interwoven and filling is covered. Trim away any loose pieces of pastry at the bottom, pinch the openings to seal it, and transfer the parchment paper with braid on it to a sheet pan. Brush the top lightly with egg white and sprinkle with pepper. Bake until pastry is deeply browned and cheese is bubbling up through the slits, about 25-30 minutes. Let cool slightly before cutting it into thick slices and enjoying warm.