In a medium saucepan combine tomato sauce, crushed tomatoes, chili powder, oregano, paprika, cumin, pepper, salt and garlic. Cover and cook over medium heat.
Meanwhile, melt butter in a small skillet over medium heat. Saute onion for about 4 minutes; stir into sauce. Cook sauce for 20 minutes, stirring occasionally. Stir in bell pepper and cook 10 more minutes.