Season the chicken thighs on both sides with salt and pepper. Heat oil in a large high-sided skillet over medium high heat. Once it shimmers, add the chicken thighs and sear until golden brown, about 4 minutes on each side. Remove to a plate. Add onion and garlic and sauté until tender, about 3 minutes. Stir in apple cider vinegar; using a wooden spoon stir up any browned bits on the bottom of the pan. Add ketchup, water, brown sugar, Worcestershire, cayenne and salt and pepper. Bring to a boil then reduce to a simmer. Add the chicken back to the pan as well as any juices that have accumulated on the plate. Turn heat to low, cover and cook for 40 minutes.
Once chicken is cooked, shred the meat with a fork in the skillet. Toss the meat through the sauce so it’s covered. Divide the pulled meat between 6 hamburger buns. Serve alongside with some coleslaw, if desired.