Heat half the oil in a large heatproof casserole dish. When really hot quickly brown the lamb, in batches, until brown. Set aside.
Add onions to the same casserole dish with the remaining oil and cook for 1-2 minutes.
Add the curry powder or paste, garlic and ginger, stir and cook for 2 minutes before adding back the lamb, tomatoes, stock, brown sugar and seasoning. Bring to the boil, cover and reduce to a simmer for an hour on a low heat, stirring from time to time.
Remove the lid, add the coconut milk and continue cooking for a further 30 minutes. Stir through the lime zest and juice. Taste and season.
Sprinkle over the coriander and cashew nuts and serve with rice and a green salad.