Preheat oven to 350 F Degrees.
Heat oil in a frying pan and brown chicken on all sides. If all of the chicken does not fit into the pan, cook in batches. Add Onion and garlic and continue to cook for about 5 minutes on medium heat. Add saffron to chicken stock and mix until dissolved. Place rice, tomato sauce, and chicken stock in a medium to large casserole pan and stir until the rice is even on the pan and well coated with stock mixture. Place chicken and Onion mixture on top. Cover with a casserole lid or aluminum foil and cook for 25 minutes or until chicken is cooked and the rice is tender. If chicken or rice has not been fully cooked by the time the liquid has been almost fully absorbed, add water in small amounts and continue to cook until done.