Heat the oil in a large, heavy-based saucepan and cook the onion over a medium heat, stirring occasionally, for 2–3 minutes until beginning to soften, then add the cauliflower, potatoes and cumin seeds. Cook for 4–5 minutes, stirring occasionally, until the potatoes are beginning to brown.
Add the curry paste and toss to coat the vegetables, then stir in the coconut milk and stock and bring to the boil. Reduce the heat, cover and simmer, stirring occasionally, for 20 minutes until the vegetables are tender, adding the spinach for the last 5 minutes of the cooking time.
Season generously with salt and pepper and stir in the coriander. Serve with warm naan breads.