Trim fat from meat. Place meat in a large resealable plastic bag set in a shallow dish.
In a medium bowl, combine onion, liquid smoke, soy sauce, Worcestershire sauce, celery seeds, garlic, and black pepper. Pour over the meat; seal bag. Marinate in the refrigerator for 6 to 24 hours, turning bag occasionally.
Place meat in a foil-lined 13x9x2-inch baking dish: pour marinade over meat. Bake in a 325 degree F oven for 3 to 3-1/2 hours or until it's very tender, turning once. Remove from oven. Carefully transfer meat to a cutting board; discard marinade. To serve, thinly slice meat across the grain. Place on a serving platter. If you like, serve in hamburger buns. Makes 8 to 10 servings.