For the croissant
Roll out the puff pastry sheet and divide it in two by cutting along the longest side with a pastry jagging wheel. From the two rectangles, obtain 12 triangles each having an 8 cm base. Gently stretch the base of each triangle. Roll it up from the base to the tip. Brush croissant tops with egg white. Preheat the oven at 195°. Place the croissants on racks covered with cooking sheets, and put them in the oven at 195° for 20-25 minutes (if you have a fan-assisted oven, at 175° for 10-15 minutes): they are ready when their surface looks golden brown and crunchy.
For the stuffing
Cut the croissants in halves, and open them. Spread the crescenza, lay a lettuce leaf and speck. Serve warm.
