Add beans, tomato sauce, and 1 cup water to pot. Add spices to taste (I usually put in 1 tsp salt, 1 tsp pepper, half tsp thyme, half tsp basil, and a shit ton of hot sauce). Let this boil down before proceeding.
Chop onion and add with rice and whatever optional ingredients you have. Fry in pan with oil and Cajun spice if you've got it. Add to pot. Let boil down until sauce is slightly thickened.