Combine first 6 ingredients in a large heavy-duty, zip-top plastic bag; add chicken. Seal bag, and shake until chicken is well coated. Marinate in refrigerator 2 hours, turning bag occasionally. Remove chicken and onion from marinade, discarding marinade. Set aside.
Coat an ovenproof Dutch oven with cooking spray; add 1 1/2 teaspoons oil, and place over medium-high heat until hot. Add 4 chicken breast halves to Dutch oven; cook 2 minutes on each side or until browned. Remove chicken from Dutch oven; set aside, and keep warm. Repeat procedure with remaining 1 1/2 teaspoons oil and 4 chicken breast halves.
Remove Dutch oven from heat; add marinated onion slices. Return chicken breasts to Dutch oven, nestling them into onion slices. Bake, uncovered, at 450° for 20 minutes. Remove chicken from Dutch oven; stir onion slices, scraping bottom of Dutch oven to loosen browned bits. Return chicken to Dutch oven; reduce oven temperature to 375°, and bake 15 additional minutes.
Spoon rice evenly onto individual serving plates, and top evenly with chicken and onion mixture.