In a large saucepan, heat vegetable oil over a medium-low heat. Place the onions in the pan, and saute until golden.
Add rice to pan, and stir to coat grains with oil. Mix in green bell pepper, cumin, chili powder, tomato sauce, salt, garlic, saffron, and water. Cover, bring to a boil, and then reduce heat to simmer. Cook for 30 to 40 minutes, or until rice is tender. Stir occasionally.