Place the leg of lamb into a dish. Mix together the salt and chilli powder and rub over the surface of the lamb. Leave to marinate for 30 minutes.
Grate the ginger and garlic and mix to form a paste. Rub the mixture all over the surface of the lamb. Place in the fridge to marinate for 12-24 hours.
Once marinated, take the lamb out of the fridge and leave to rest for 30 minutes so it reaches room temperature. Heat your oven to 180°C or fan 160°C.
Place the lamb into a deep casserole dish that has a lid. Add the bay leaf, all the spices and enough water so that it fills at least halfway up the lamb (approx 300ml). Put the lid on the dish and place in the oven to cook for 4 hours. Each hour, turn the joint over and baste with water. Should the water dip below the bottom third of the lamb leg, top it up with some more.
After the 4 hours, remove from the oven and leave to rest for 30 minutes. Heat your grill to high.
Once rested, remove the lamb leg from the pot, and pour the cooking liquid through a sieve into a jug for later. In a dish, shred the lamb using two forks.
Add the butter, kebab masala and 120 ml of the cooking liquid to the lamb. Place under the grill for 5 minutes until some of the lamb goes crispy.
Sprinkle on extra kebab masala and squeeze on the juice of a lime. Serve with Dishoom's Gunpowder Potatoes.