Heat oil in sautée or sauce pan (2–3 quart size), add onion and cook until softened, stirring with a wood spatula, about 5 minutes. Add the rice and stir to toast.
Add salt and tomato option, stir to combine. Add 3 ½ cups water or broth and bring to a boil. Lower heat to maintain a slow simmer, cover and let cook until liquid is absorbed into rice, about 20 minutes. Turn off heat and let sit 5 minutes (or up to 1 hour) before serving.