Prepare barbecue sauce. To butterfly loin, split meat lengthwise almost all the way to opposite side, then spread open flat. Pound out to a 15x10 inch rectangle, about 2 cm thick. Brush top with quarter cup of the barbecue sauce.
Combine ground beef, onion, garlic, salt, pepper and an additional quarter cup of the barbecue sauce. Spread evenly over roast. Press mushrooms into ground beef. Sprinkle bread crumbs and parmesan cheese over meat. Starting from 3 cm side, roll up and tie. Place on rack in shallow roasting pan. Roast, uncovered, in 325 degrees oven for about 2.5 hours. Baste with additional barbecue sauce during last 15 to 20 minutes of roasting.
Pass remaining barbecue sauce. Garnish with celery leaves, if desired.
Makes 6 to 8 servings.
Barbecue Sauce: In medium saucepan combine ketchup, chili sauce, wine vinegar, packed brown sugar, lemon juice, worcestershire sauce, steak sauce, mustard, salt, pepper and garlic. Simmer 30 minutes. Remove from heat.
Makes 2.6666666666667 cups sauce.