1. Trim fat from brisket to 1/8-inch thickness.
2. Combine celery salt, garlic powder, and onion salt. Sprinkle brisket evenly with mixture, and rub thoroughly into meat.
3. Stir together barbecue sauce and next 3 ingredients until blended. Brush barbecue sauce mixture evenly onto brisket. Wrap brisket in heavy-duty aluminum foil; place in a roasting pan. Chill 8 hours.
4. Bake at 300° for 5 hours or until a meat thermometer inserted in thickest portion registers 190°. Let stand 10 minutes. Cut brisket across the grain into thin slices. Garnish, if desired.