Cut each one of the lamb shanks into 2 pieces (your local butcher can do it for you). This makes it easier to eat the bone marrow which is very delicious rather than keeping them whole. Then clean them by trimming away the fat and removing any unwanted thick skin carefully without chopping off the tender meat. Rinse well, drain and keep aside.
Slice the two onions. Heat the vegetable oil in an Instant Pot/pressure cooker, add the black cardamom, cloves and the bay leaf and fry half the onion slices to a light golden brown. Reserve the other half. Use the sauté setting in the Instant Pot for this.
Add ginger garlic paste, turmeric, kashmiri chilli powder and coriander powder, give it a stir then add the lamb shanks and salt. Sauté well with the spices.
Then spoon in the whisked yoghurt to the Instant Pot followed by green chillies, cilantro and 1-2 cups of water. The water should be 2 inches above the shanks. Switch off the sauté mode, close the lid, turn the valve to sealing position, then press the "pressure cook" or the "meat/chilli" button and set the time for 20 minutes.
In the gas pressure cooker after the first whistle, turn down the heat to medium, close the lid air tight and pressure cook for 10 - 15 minutes. (But if you are not using either the Instant Pot or pressure cooker you will have to cook on low to medium heat for at least 45minutes to an hour on the stove. They should be so tender as to come off the bone. If the meat is still not soft and off the bone in the Instant Pot or Pressure cooker, cook for 5 - 10 more minutes. It all depends on the type of meat. Normally the shanks cook in 20 - 30 minutes in the Instant Pot and 10 - 20 minutes in the gas Pressure Cooker.
Meanwhile deep fry the reserved onion in a frying pan in vegetable oil till evenly and nicely browned but not burnt. So extra care should be taken while frying the onions to have them rightly done. Set aside.
Once the shanks are done, wait for NPR and if you have no patience do a QPR (manually release the pressure by turning the valve to venting position) then switch to sauté less mode. Once the pressure cooker time is done, switch off the stove and let out the steam naturally or manually.
Now add the reserved fried onions to the Instant pot or your gas Pressure Cooker, cover lightly not air tight and let it simmer slowly for some more time (5-10 mins). The browned onions not only give colour to the curry but also a sweet nutty flavour which combined with the smoky black cardamom flavour further enhances the taste.
Do not let the curry thicken too much. If it tends to thicken then sprinkle a little water on all sides and let it simmer till the oil comes on top or until you don't see those fried onions anymore. Add chopped fresh cilantro to garnish and enjoy with rotis (flat bread), naan or rice.