Heat half the oil in a large, heavy-based saucepan and brown the lamb, in batches, for 3–4 minutes. Remove with a slotted spoon and set aside.
Heat the remaining oil in the pan and add the onion, garlic, ginger, turmeric, nutmeg, sultanas, cinnamon and paprika. Stir-fry for 1–2 minutes and then add the lamb. Stir-fry for 2–3 minutes and then add the tomatoes and stock. Season well and bring to the boil. Reduce the heat, cover tightly and simmer very gently (using a heat diffuser if possible) for 1½ hours.
Add the spinach in batches until it is all wilted, cover and cook for a further 10–12 minutes, stirring occasionally. Remove from the heat and serve drizzled with whisked yogurt, if liked.