Heat a pan of boiling water and cook the potato until almost tender, about 5 minutes, then drain well. While it’s cooking, put the onion, garlic and ginger in a narrow container with 2cm of water and use a stick blender to blitz until smooth (or whizz in a small food processor). Put 6 tbsp oil in a heavy pan, heat, add the potatoes and fry until golden. Remove and set aside. Fry the cauliflower until golden. remove and set aside. Put the cumin in the pan, cook for 30 seconds, add the onion mix and cook for 4-5 minutes. Add the remaining spices, chillies and tomatoes, cook for 2 minutes, then add the potato and cauliflower with 500ml water. Cook for 4 minutes. Scatter with coriander and serve with yoghurt and rice or naans.