Pre-heat oven to 375 degrees.
De-bone turkey meat, leaving each side of the breast as much intact as possible and leaving the skin on as well. (don’t throw the bones away, roast them and use as a base for soup or something!).
Butterfly out each half of the turkey breast and then cover with plastic and pound with a mallet or the back of a large frying pan until about ½ inch thick evenly across. Set aside chilled.
Wash and trim the asparagus and steam just a few minutes until bright green. Run them under cold water to stop cooking and set aside.
Boil and mash about 2 cups worth of potatoes. Mash into them the butter, cream, cheese, white pepper, egg and truffle oil until smooth with no lumps.
Lay the turkey breasts out skin side down on top of a piece of plastic wrap. Sprinkle with a little salt and pepper. Cover with a thin layer of the prosciutto, then the emmentaler cheese. Then spread about 1 cup of the mashed potatoes on each half and top with the asparagus. Using the plastic wrap to help pull up each side and roll the turkey breast together (from the wide end not the narrow end) Use kitchen string to tie in a few places, place in roasting pan. Repeat with the second breast, layering and tying up. Place in roasting pan as well. Roast covered for the first 20 minutes then uncover and add ½ cup white wine or broth and continue to cook until cooked through, (about another hour) Deglaze the bottom of the pan with the broth while the turkey is resting and make your favorite gravy from the drippings.