Chop the onion into small, uniform pieces and mince the garlic finely. Spray your slow cooker generously with non-stick cooking spray, then spread the chopped onion across the bottom in an even layer.

In a medium bowl, whisk together the BBQ sauce, apple cider vinegar, Worcestershire sauce, brown sugar, smoked paprika, garlic powder, onion powder, and black pepper until completely smooth with no clumps remaining.

Arrange the chicken breasts on top of the onion layer without overlapping them too much. Pour the BBQ sauce mixture evenly over each breast, making sure every piece gets thoroughly coated.

Cover the slow cooker with its lid and set to low for 6 to 8 hours or high for 3 to 4 hours. The chicken is ready when it shreds easily with a fork and reaches an internal temperature of 165°F using an instant-read thermometer.

Remove each chicken breast carefully and place on a cutting board. Use two forks to shred the meat into bite-sized pieces, pulling the forks in opposite directions. Return all shredded chicken to the slow cooker and stir into the sauce.

Let the shredded chicken simmer in the sauce for another 15 to 20 minutes to allow flavors to penetrate. Leave lid off to thicken sauce or keep on to maintain moisture. Serve hot on hamburger buns topped with coleslaw or as desired.