Trim the asparagus stalks of their woody bottoms and, if they have thick skins, peel them below their tender tips. Boil, or better steam, the asparagus until tender but still a bright green color. Refresh the asparagus under cold running water to stop it cooking.
Lay the puff pastry out flat on a large piece of parchment paper.
Lay down the prosciutto slices on top of the pastry, leaving a border of at least 3cm/1 in on all sides. Lay the sliced cheese on top of the prosciutto. Finally, lay down the asparagus spears on top of the cheese in parallel rows.
Now, tuck in the tops and bottoms of the pastry and, using the parchment paper to help you, roll up the puff pastry carefully horizontally, ending up with the seam side down.
Place the strudel, still cradled in the parchment paper, in a baking dish, cut parallel incisions across the top, then brush it all over with the beaten egg:
Place in a hot 200C/400F oven for about 30 minutes, or until nicely puffed and golden brown on top.
Let the strudel salato cool completely before serving.
Let the strudel salato completely before serving.