Scatter onions on bottom of slow cooker. Arrange chicken on top of onions.
In a bowl, combine ketchup, vinegar, mustard, molasses, onion powder, cumin, garlic powder, Tabasco and salt. Pour over chicken. Cover and cook on low until chicken is fork-tender and sauce has thickened, about 5 hours.
Remove chicken from slow cooker and shred meat. Stir back into sauce. Serve hot, on whole-wheat buns, if desired.