Heat oil in a 12" skillet over medium heat; cook chicken, turning to brown all sides, 3 to 4 minutes per side. Remove chicken from skillet and set aside. Add onion to skillet and cook for about 2 minutes.
Add diced tomatoes with peppers, tomato sauce, water and Spanish-style rice mix to the same skillet; stir well. Return chicken to the rice mixture and bring to a boil. Reduce heat to low, cover skillet, and simmer until chicken is no longer pink in the center and rice is tender, about 40 minutes.
Serve with a scoop of sour cream. grated cheese and a sprinkle of green onion.