In a large pot (4 or 5 quarts), melt butter over medium heat and add onions.
Cook, stirring often, about 5-6 minutes, until softened.
Add bell pepper, celery, and garlic. Cook 5-6 more minutes, stirring occasionally, until the vegetables are tender.
Add remaining ingredients (don't add the cooked chicken, if using, you'll add it near the end), stir well, and bring to a low boil, reduce heat, and simmer, uncovered, for about 40-45 minutes, until the potatoes are tender and chicken is cooked through. If using cooked chicken, stir it in at this point and let it heat through.
Discard bay leaves and serve.
