- Place white vinegar in a bowl. Crush garlic, peel and chop finely. Add garlic to bowl with chilli powder, salt and ground white pepper.
- Place cumin and coriander seeds in a small grinder or mortar. Grind very finely.
- Peel root ginger and chop finely. Stir into vinegar mixture with seeds, garam masala, food colouring and yoghurt.
- Slash lamb in several places. Place in a large bowl. Pour over marinade. Cover and refrigerate for 24 hours.
- Place lamb on a roasting rack in an oven dish, with ½ cup of water covering the base of the dish.
- Bake at 200 degrees C for 1½ to 2 hours. Stand for 10 minutes before carving. Serve with naan bread and vegetables.