Carefully clean the leeks, cut away the roots and cut into thirds. Cook in salted water for eight to ten minutes, depending on thickness, until tender. Carefully lift out and rinse under cold water. Dry well with kitchen roll.
While the leeks are cooking, grate the cheese on a rösti grater.
On each slice of ham form 1 leek piece, sprinkle with cheese and roll up tightly.
Spread the puff pastry on the surface and cut into rectangles (1 rectangle for two leek-ham rolls). Spread the leek-ham rolls evenly on top, trim the dough if necessary, brush the edges with egg white and close everything together to form packets. Place them seam side down on a baking sheet lined with parchment paper. Mix egg yolk, cream and salt and brush the leek rolls with it. If possible, allow to cool for fifteen minutes before baking.
Bake the leek rolls in the oven heated to 200 °C on the second rack from the bottom for twenty to twenty-five minutes until golden brown. Serve hot with a green leaf salad (e.g. arugula).
Tip: Always use an aromatic ham, it will give a wonderful touch to your dish!