Barbecued Chicken

3 lb chicken parts (breast, drumstick, and thigh), skin and fat removed
1 large onion, thinly sliced
3 Tbsp vinegar
3 Tbsp Worcestershire sauce
2 Tbsp brown sugar
to taste black pepper
1 Tbsp hot pepper flakes
1 Tbsp chili powder
1 C chicken stock or broth, fat skimmed from top