Heat broiler or grill. Spray rack. Spray chicken pieces. Broil 30 minutes, turning often, and spraying again at each turn.

Heat butter in saucepan. Add onion and saute until limp. Sprinkle with flour and cook, stirring, 2 to 3 minutes. Remove from heat and add boiling broth, beating vigorously with wire whisk to prevent lumping. Return to heat and cook, stirring, until smooth and thick.

Stir in tomato paste, Worcestershire sauce, sugar, and mustard. Serve over broiled chicken.