Preheat oven to 425 degrees F. Trim fat from meat. Make a lengthwise cut down the center of the meat, cutting almost to, but not through, the other side. Spread open; set aside.
For stuffing, in a large skillet cook pancetta in hot oil over medium heat until crisp. Using a slotted spoon, remove and drain pancetta on paper towels, reserving drippings in skillet. Add the 3 cups mushrooms, the shallots, and garlic to reserved drippings. Cook until mushrooms and shallots are tender, stirring occasionally. Cool slightly. In a large bowl combine 1 1/2 cups of the bread crumbs and the parsley. Add pancetta and mushroom mixture; toss gently to combine.
Spoon stuffing down the center of the meat. Bring up sides of meat around stuffing, pressing to almost enclose stuffing. Tie at 2-inch intervals with 100-percent-cotton kitchen string. Place roll, seam side up, on a rack in a shallow roasting pan.
In a medium bowl combine butter, cheese, and mustard. Spread evenly over top and sides of meat roll. Press the remaining 1 cup bread crumbs onto the top and sides of roll.
Roast, uncovered, 40 to 45 minutes for medium-rare (140 degrees F). Remove from oven. Cover loosely with foil; let stand 10 minutes before slicing. (The temperature of the meat will rise 5 degrees F while it stands.)
Remove string. Cut roll into 1/2-inch slices. Serve warm. If desired, serve with sauteed mushrooms.