In your slow cooker, combine the onion, ketchup, apple cider vinegar, brown sugar, tomato paste, paprika, Worcestershire, salt and pepper. Whisk to combine.
Add the chicken thighs and make sure they are all evenly spread out and coated with the sauce ingredients.
Place the lid on the slow cooker and turn to high heat for the Shredded BBQ Chicken to be done in 4-5 hours, and low for Shredded BBQ Chicken to be done in 7-8 hours.
Once the timer is up, remove the lid and using 2 forks, shred the chicken while still in the crock pot. Turn the crock pot to warm and serve the Shredded BBQ Chicken on top of burger buns with a heaping spoonful of guacamole and shredded colby jack cheese.
Notes
Alternatively - you could do this in the instant pot! Same directions but high pressure for 20 minutes. Leave it alone for 10 minutes and then release pressure and shred with forks as needed.