Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a large frying pan, add the beef and onion, breaking up and stirring the meat until it is evenly browned. Stir in the curry paste and turmeric and cook for 1 minute. Stir in the carrots and flour, then add the lentils, sultanas, tomato purée, stock and salt and pepper. Bring to the boil, stirring, then pour into the slow cooker pot. Cover with the lid and cook on low for 8–10 hours or until the lentils are soft and the beef is tender.
When almost ready to serve, put the potatoes in a saucepan of boiling water and simmer for 15 minutes or until tender. Drain and mash with half the butter, milk and salt and pepper. Heat the oil in a frying pan, add the spring onions and fry for 2–3 minutes or until softened. Add the turmeric and cook for 1 minute, then mix into the mash.
Stir the beef mixture and lift the pot out of the housing using oven gloves. Transfer to a serving dish if liked. Spoon the mash on top, dot with remaining butter, then grill until golden. Serve with cooked peas.