Prepare the mutton: Cut the mutton into medium sized pieces and wash it well. If your mutton has enough fat, you won't need any oil. If it's lean, add a little oil or ghee to the pan.
Cook the mutton: Place the mutton in a large cast iron wok or karahi over medium heat. Cook for 30 to 40 minutes, stirring occasionally, and the mutton starts releasing its own fat.
Add the aromatics: Add ginger, garlic paste, and salt, stir well for a few minutes. Add the tomatoes cut into halves along with 1 cup of water. Cover and cook for about 40 minutes, until the tomatoes turn completely soft.
Mash the tomatoes: First, remove excess oil from the karahi. Then, remove the tomato skins with tongs, and mash the tomatoes with a spatula. Cook on medium heat until the extra liquid evaporates and the gravy thickens.
Finish the curry: Add the chopped green chilies and black pepper. Cook on low heat for 10 minutes, until the mutton is tender and coated in a rich, juicy masala.
Serve: Garnish with julienned ginger and serve hot with roti or steamed rice.