Boil the potatoes and let them cool. Soak the cauliflower in cold water for 30 minutes. Then drain. Cut the potatoes into 20 mm cubes. Cut the cauliflower into flowerettes (about 40 mm across).
Heat the oil in a wok and add the whole cumin seeds. Stir fry until they begin to pop (a few seconds). Add the cauliflower and stir fry for about 3 minutes. Cover and simmer for a further 3 minutes. Add the potato cubes, the chilies, ground cumin, ground coriander, turmeric, chili powder, salt and pepper and mix well without breaking up the potatoes. Cook covered for a further 5 minutes. Sprinkle on the chopped coriander prior to serving.