Heat the oil in a heavy-based saucepan or the base of a tagine, stir in the onions, garlic, ginger, chillies and cumin seeds and cook for 2–3 minutes. Add the turmeric and paprika and toss in the potatoes and carrots.
Pour in the stock and bring to the boil. Cover, reduce the heat and cook gently for 15 minutes until the vegetables are tender.
Stir in the peas and half the herbs. Season and top with the tomato slices. Scatter over the butter and sprinkle with the remaining herbs. Re-cover and cook for a further 5–10 minutes. Serve with couscous, if liked.