Place chicken, onion, and BBQ sauce in the slow cooker. Cook on high for 3-4 hours or on low for 6-8 hours.
Remove the chicken from the slow cooker and place on a large cutting board or platter. Using two forks, shred the chicken. Place the shredded chicken back in the slow cooker and stir until chicken is well coated in the sauce.
Serve the BBQ chicken in buns or rolls with cabbage or slaw, if desired. You can also use the BBQ chicken to make pizza, quesadillas, salads, tacos, and more!