1. Wipe lamb, trim and cut into cubes.
2. Put lamb in a glass or china bowl.
3. Mix together turmeric, cumin, coriander, ginger and garlic and add to lamb.
4. Stir well, then cover loosely and leave in a cool place for 2-3 hours.
5. Heat oil in heavy-based pan until smoking.
6. Stir in sugar, then add onions and cook over a medium to high heat for 10 minutes, stirring frequently, until a rich brown.
7. Remove onions with a slotted spoon and set aside.
8. Add lamb to pan and fry until browned all over.
9. Add potatoes and fry, stirring, for 2 minutes. Return onions to pan, add 250 ml (8 fl oz/1 cup) water and season with salt and cayenne pepper.
10. Bring to the boil and simmer, covered, for 1 1/4 hours, or until lamb is tender, stirring occasionally.
11. Stir in garam masala and serve, garnished with rosemary spigs.

Serves 4.