Heat the oil in a large heavy-bottomed pot over medium heat until hot.
Add the sliced onions and cook for 6–8 minutes, stirring occasionally, until they turn golden brown. Remove the onions and set them aside.
Place the lamb pieces into the same oil and cook for about 6–8 minutes, turning occasionally until the meat changes colour and starts to brown slightly.
Return the fried onions to the pot, then stir in the ginger paste and garlic paste. Cook for 2–3 minutes to remove the raw aroma.
Sprinkle in the salt, Kashmiri chilli, and Nihari masala. Stir well and let the spices cook for 2–3 minutes until the oil begins to separate from the masala.
Pour in enough water to cover the meat completely, then mix thoroughly.
Cover the pot with a tight-fitting lid and simmer over low heat for about 2 hours, stirring occasionally, until the meat becomes tender.
Once cooked, carefully remove any excess oil that rises to the top and set it aside for later use as garnish.
In a separate bowl, mix the flour with water to create a smooth slurry with no lumps. Gradually pour the slurry into the Nihari while stirring continuously to prevent clumping.
Let it cook uncovered for 10–15 minutes on low heat, stirring often, until the gravy thickens to your desired consistency.
Finish by garnishing with the reserved oil, fresh coriander, ginger julienne, green chillies, and a wedge of lemon before serving hot.
