Preheat oven to 450°.
Combine the first 7 ingredients in a small bowl. Rub both sides of chicken with spice mixture. Arrange onion and bell peppers in a 13 x 9-inch baking dish, and top with chicken. Bake at 450° for 20 minutes. Reduce oven temperature to 375° (do not remove chicken from oven), and bake an additional 15 minutes or until chicken is done. Remove chicken from dish, and keep warm. Remove onion mixture from dish, and chop. Combine onion mixture, basil, nuts, and vinegar. Serve with chicken.