In a small pan, dry roast all the ingredients(listed under spice blend)for Nihari masala, for few minutes. Let them cool and transfer them to a grinding jar. Blend it to a fine powder and keep aside.
Switch on the instant pot and set it on for "saute" mode. If not using Instant pot, then heat a heavy bottomed Kadhai or handi over medium flame.
Add ghee to it and once ghee is warm, add grated ginger and garlic. Saute for 1-2 minutes until raw smell fades away. Add lamb shank pieces and saute for 4-5 minutes.
Add Kashmiri chilli powder, salt and the grinded Nihari masala to it and saute for 2 minutes till spice mixture is roasted well and lamb shanks are coated nicely and switch off the instant pot. If cooking in handi, then reduce the heat to lowest.
Add 8 cups of water(if using instant pot, otherwise 10 cups if cooking in handi/kadhai) and mix well. Close the instant pot with lid and adjust the setting to "slow cook" mode for 2.5 hours. If cooking in handi, then cover the handi with lid and let it cook on lowest heat for 2.5 hours.
After that, open the lid and check if the shanks are cooked nicely. It should be really tender once you touch it with the spoon and comes off the bone easily. Now adjust the instant pot to "saute" mode again.
Sieve together all purpose flour and wheat flour. Take it in a bowl and add 1/2 cup of water. Mix well and add this to the cooking Nihari. Saute for 2-3 minutes so that there are no lumps formed and let Nihari cook for 10-12 minutes more.
Heat another pan over medium flame and add oil to it. Once warm, add cumin seeds and let it crackle. Add sliced onions and saute till they are golden. Add chopped green chillies and ginger julienne and saute for 1 minute.
Switch off the heat and transfer this tempering to cooking Nihari. Mix well and switch off the heat.
Transfer to a serving bowl and garnish with chopped coriander and lemon wedges. Serve hot and enjoy with Naan or rice.