Boil mutton with ginger, garlic and green chili paste for 15-25 minutes until half cooked.
Simultaneously, eat oil or ghee in a karahi or wok and add onions (if using). Fry until onions are translucent then add all the ingredients in the spice list. Fry for a minute and let the spices sizzle. Immediately, add the tomato puree. Cover and cook the gravy for 5-7 minutes on medium heat.
Then, remove the lid and dry out excess water with constant stirring until you see a little oil shining on the sides of the pan. (Drying excess tomato water is essential it gives the karahi its yummy taste.)
Now, add the remaining water from boiled meat to the tomato gravy. Remove and set aside.
Add 1 tablespoon oil in the meat pot and stir fry meat on high flame until golden for 2-3 minutes.
Now add the meat to the tomato gravy. Also add fenugreek leaves, green chilies, and half of the julienned ginger. (At this stage, adjust gravy to your preference and little water only if needed.)
Cover and cook for another 10-15 minutes on medium-low heat until meat is very tender and gravy has reached the desired consistency.
Finally add a little butter on karahi and serve. Garnish with the remaining ginger and coriander leaves. Serve with lemon wedges.