Place the cooked chicken breasts, bbq sauce and water in a slow cooker lined with a slow cooker liner on low heat for 8 hours. Once cooked, pull apart the chicken using two forks to shred it.
Next, in a medium size saute pan on medium high with butter, caramelize the onions (this can take up to 30 minutes to do so). After 10 to 15 minutes of cooking the onions add in the sugar to help sweeten and caramelize. After 20 to 25 more minutes of cooking, deglaze with the onions with balsamic vinegar and cook until the balsamic vinegar is almost gone. Once cooked, set aside.
In a large bowl mix together the mayonnaise, vinegar, celery seed, salt and pepper and toss in the chopped coleslaw mix (you will have to chop the coleslaw mix as it comes to you shredded).
To Serve: Place the bbq pulled chicken on a toasted bun and top off with the caramelized balsamic onions and a tablespoon or two of creamy coleslaw.