In a wok, fry onion on slow heat for 7-10 minutes. When onion slices are very soft increase heat to medium and stir constantly until lightly golden.
Mix in ingredients in the spice list and give it a stir. When spices sizzle adds tomatoes. Cover and cook on medium heat for 5-7 minutes. When tomatoes are soft stir on high heat and dry all tomato water.
When oil separates on sides of wok pour in yogurt and fenugreek leaves. Sear for another 2 minutes.
Now, mix in liver pieces and stir for a minute until color changes. Cover and cook for 2 minutes on medium flame. Remove lid, sprinkle salt. Cook for another 2 minutes on high heat with constant stirring.
Finally, remove from heat and cover the pot and rest it for 10 minutes at least. The steam in the pot will cook the liver completely.
Reheat on high flame and and add butter, green chilies, green corianders and ginger. Burn any excess water and sear until oil seprate at the side of pan. (Bhuno for 1-2 minute).
Serve immediately with lemon wedges.