Heat the oil in a heavy-base, medium-sized pot over medium heat. Add the cumin seeds and cook for 10 seconds, until aromatic (keep a lid handy to catch any seeds that might fly out). Add the carrot, fennel, celery, potato and beans, and stir-fry for 5 minutes. Add 8 cups stock and salt. Bring to a boil, reduce heat, partially cover and simmer over medium heat for 1 ½ hou