In a saucepan, combine the first 8 ingredients. Mix well. Heat. Add butter and melt. Simmer for 5 minutes.
Let cool at room temperature. Arrange onion and the chicken breasts in a single layer in a shallow glass or baking dish.
Pour on the marinade and let stand, covered, for an hour or more in the refrigerator.
Cook the chicken about 7 to 8 inches from the broiler heat for about 10 minutes on each side, turning several times during the cooking process.