The inspiration for this crust was the Italian cheese wafer frico, made by baking or frying shredded cheese until crisp.
Tear two 12-inch-long sheets of wax paper. On each sheet, arrange half of the prosciutto slices to form a 12-by-6-inch rectangle, overlapping the slices slightly. Transfer to 2 large, flat cookie sheets.
In a bowl, combine the herbs with the panko, grated cheese and 3 tablespoons of the oil; season with salt and pepper. Pat the herbed crumbs evenly over the prosciutto. Place a sheet of plastic wrap on top of each rectangle and, using a rolling pin, press the crumbs onto the prosciutto. Freeze the prosciutto crusts for 10 minutes, until firm.
Preheat the oven to 425°. Cut the meat into 2 even roasts and season with salt and pepper. In a large skillet, heat the remaining 1 tablespoon of oil. Add the roasts and cook over moderately high heat until lightly browned all over, about 6 minutes. Transfer to a baking sheet and let cool.
Add 2 tablespoons of the butter to the skillet with the shallots and cook over moderate heat until they are softened, 3 minutes. Add the port and simmer over moderately low heat until syrupy, scraping up any browned bits, 6 minutes. Add the demiglace and simmer until slightly reduced, 5 minutes. Swirl in the remaining 2 tablespoons of butter.
Invert the unbaked prosciutto crusts onto a work surface and gently peel off the wax paper. Place the crusts over the roasts, prosciutto side down. Discard the plastic wrap. Roast the meat for about 40 minutes, or until an instant-read thermometer inserted in the thickest part registers 130° for medium-rare. Let the roasts rest for 20 minutes.
Meanwhile, remove the crusts from the roasts and place on a baking sheet, crumb side up. Press to flatten and bake for 6 to 7 minutes, just until golden and the prosciutto is crispy. Let cool, then break into shards. Rewarm the sauce. Cut the roast into slices and serve with the crisps and sauce.