Boil the potatoes in their skins and allow them to cool completely. Peel the potatoes and cut them into 2 cm dice.
Break the cauliflower into chunky florets about 4 cm across at the head. Soak in a bowl of water for 30 minutes then drain.
Heat the oil in a large non stick frying pan over a medium heat. When hot put in the whole cumin seeds and sizzle for 3-4 seconds. Add the cauliflower and stir for about 2 minutes, allowing to brown in spots.
Cover, turn heat to low and simmer for 4-6 minutes or until the cauliflower is almost done but still a bit crisp.
Add the diced potatoes, ground cumin, coriander, turmeric, cayenne, green chilli, ground roasted cumin, salt and some black pepper. Stir gently to mix.
Continue to cook, uncovered on a low heat for 3 mins or so until the potatoes are heated through, stirring gently. Serve immediately.