In a small bowl combine the chili powder, celery seeds, salt, and pepper. Rub the spice mixture onto all sides of the brisket. Scatter half of the sliced onions in the bottom of a 3 1/2-, 4-, 5-, or 6-quart crockery cooker. Place the brisket on the onions, cutting the meat to fit the cooker, if necessary. Scatter the remaining onions on top of the brisket. In a small bowl stir together the barbecue sauce and beer or ginger ale. Pour over the brisket and onions.

Cover and cook on low-heat setting for 10 to 12 hours or until meat is fork-tender. Transfer meat to a cutting board and let stand for 15 minutes. Halve meat crosswise. Using 2 forks, pull meat apart into shreds. Return meat to sauce mixture in crockery cooker. Heat through using the high-heat setting.

To serve, use a slotted spoon to transfer beef and onion mixture into the buns. If desired, season to taste with bottled hot pepper sauce. Top with mango slices. (Freeze any remaining beef mixture in a freezer container for up to 3 months.) Makes 8 servings.

Preheat oven to 325 degree F. Combine the seasoning mixture and rub onto brisket as above in Step 1. Place meat in a shallow roasting pan. Top with all of the sliced onions and pour a mixture of the barbecue sauce and beer over meat. Cover and roast about 3 hours or until meat is fork tender. Remove meat and let stand 15 minutes. Halve meat crosswise and shred. Pour sauce into a large saucepan; add shredded meat. Heat through. Serve as directed above in Step 3.