Method:
Heat the oil in a large pot over medium heat. Add onions, carrots, celery, peppers and garlic and cook for 10 to 12 minutes, until vegetables are softened but not browned. Add chipotles, oregano, cumin, chili powder and salt. Stir to blend. Add tomatoes and 4 cups water. Gently simmer over low heat, uncovered, for 45 minutes. Add beans and simmer an additional 30 minutes. Garnish individual bowls with green onions or sour cream, if desired.