Make barbecue sauce: In a large bowl, combine ketchup, brown sugar, water, apple cider vinegar, Worcestershire, molasses, and spices. Reserve 1 cup of sauce for later. Add chicken to remaining barbecue sauce. Cover and refrigerate for at least 1 hour and up to overnight. Make chicken: Preheat oven to 425° and line a large baking sheet with foil. Place onions on baking sheet, drizzle with oil, and toss to coat. Place chicken on top of onions, skin side up, and bake until golden and internal temperature reaches 165°, 30 to 40 minutes. Turn oven to broil. Brush chicken with reserved barbecue sauce on both sides and broil until skin is crispy and sauce is thickened, 5 minutes.