Peel off the orange rind making sure you leave the white pith behind. Cut the rind into very fine 2 1/2 julienne strips. Combine the turmeric and salt with 1 1/2 litres water in a pan and bring to the boil. Pour half into a measuring jug and reserve.
Add the rind to the boiling liquid in the pan and boil rapidly for 1 minute. Empty the pan through a sieve. Pour the reserved turmeric water back into the pan and return to the boil.
Put the rind back into the pan and boil again for a minute, then again strain through a sieve. Rinse the rind under cold running water and set aside.
Pour the oil into a large non stick lidded pan over a medium high heat. When the oil is hot add the onions and fry for 6-7 minutes or until reddish brown.
Add the cumin and coriander and stir fry for 30 seconds then add the ginger and garlic and stir and fry for a minute. Add the yoghurt, a tablespoon at a time and stir it in, making sure it is absorbed before adding more.
Add the meat, turmeric and cayenne pepper. Stir and cook for about five minutes, breaking up all the lumps in the meat. Add the orange juice, orange rind and salt. Stir and bring to a simmer.
Cover, reduce the heat to low and simmer gently for 40 minutes. Add the chillies, coriander, mint and garam masala. Stir to mix. Cover and continue to cook gently for a further 10 minutes.