Begin by cleaning the mutton kidney thoroughly under running water. Once cleaned, chop the mutton kidney into small pieces and set aside.
Heat a kadai or a deep frying pan on medium heat and add a tablespoon of butter or oil.
Once the butter is slightly hot, add the chopped onions and fry until golden brown.
Add the ginger and garlic paste and fry for a minute until fragrant.
Now add the chopped mutton kidney and mix well. Add the fresh garam masala, chili powder, and turmeric, and stir to combine.
Cover the kadai and let the mutton kidney cook for about 10-12 minutes on low heat. Make sure to stir occasionally so that it doesn't burn.
Once the mutton kidney is well cooked, add salt to taste and mix well. Cook for another minute.
Garnish with chopped green chilies and fresh coriander.
Dish out the mutton kidney onto a plate or a serving dish. Serve hot with rice, dal, or roti, or just have it as a snack.