1. Make the mash. Cook the sweet potatoes in a pan of boiling, lightly salted water for 15 minutes, until tender. Drain well, mash with the soured cream, seasoning and chives.
2. Meanwhile, heat the oil in a large frying pan over a medium heat. Add the onion, garlic, chilli and ginger and cook, stirring occasionally, for 6-8 minutes, until golden. Add the cumin seeds, cook for 1 minute, then stir in the tomatoes, mince and curry paste and cook for 5 minutes, until the tomatoes are pulpy. Add the stock and simmer for 10 minutes.
3. Preheat the oven to 200°C/fan 180°C/gas 6. Stir the peas and lime juice into the keema mix and season. Spoon into an ovenproof dish and top with the mash, roughing it up slightly.
4. Transfer to a baking sheet and bake for 20 minutes, until the top is lightly browned. Serve with mango chutney.