Skin chicken. In a large skillet heat oil. Add chicken to skillet. Cook, uncovered, over medium heat for 10 minutes, turning occasionally to brown evenly. Transfer chicken to a 2-quart rectangular baking dish; set aside.
Add the onions and celery to skillet; cook until tender, about 5 minutes. Meanwhile, in a medium bowl combine ketchup, brown sugar, lemon juice, Worcestershire sauce, and mustard. Add to skillet; bring to boiling. Remove from heat; spoon over chicken in baking dish.
Bake chicken, covered, in a 375 degree F oven for 40 minutes. Uncover; spoon sauce over chicken. Bake, uncovered, for 10 to 15 minutes more or until chicken is tender and no longer pink, spooning sauce over chicken once or twice. Transfer chicken to serving platter. Skim fat from sauce; pass sauce with chicken. Makes 6 servings.