This is my grandma's recipe and my mom used to make it all the time. It's so good and the chicken turns out very tender and just falls apart (as evidenced below in the picture). This is definitely a more tangy sauce and the onions really create a great flavor.
Preheat oven to 350 degrees. In stock pot mix together water, vinegar, brown sugar, mustard, salt, pepper, onion, and butter. Bring to a boil and let boil for around 10 minutes. Then add ketchup and worcestershire sauce and mix together. Add pieces of chicken and try to cover the chicken w/ the sauce as much as possible (it won't be completely submerged).
Place stock pot in oven and cook for an hour checking chicken halfway through to turn and recoat with sauce.