Methods 1) Heat the oil in a frying pan. Add the onions, carrots and celery and cook for 5 minutes until softened. Add the garlic, cayenne, cumin seeds, oregano and thyme and stir for 1 minute. 2) Add the tomato puree to the frying pan and stir well. Pour the vegetables into a slow cooker. Add the beans and vegetable stock and stir. 3) Cook on low for 8 hours. Just before serving, stir in the lime juice. Garnish with sour cream if desired. Serves 6-8.