Select Sauté on the Inner Pot and set to the highest heat. Season each shank with a pinch of salt.
Add a drizzle of oil into the inner pot and add the lamb shanks. Brown the shanks on all sides until golden. If your shanks are rather big, do this in two batches or do all 4 in a large frying pan instead.
Once the lamb is brown, cancel the Sauté setting and add the garlic, lemon juice, tomato paste, tomatoes, bay leaves, oregano (reserve some for serving), and water into the inner pot. (If you browned your lamb in a saucepan, add them to the inner pot).
Secure the lid, select Pressure Cook and set the timer to 30 minutes. When the cooking program has finished, perform a quick pressure release.
Remove the lid and add the potatoes to the inner pot, around the lamb. Secure the lid again, select Pressure Cook and set the time to 4 minutes. Once the timer is up, allow a natural pressure release for 10 minutes. Test the potatoes are cooked through by piercing them with a knife. Add another minute of pressure cooking if they are still a little hard.
Season the Greek lamb to taste with salt and pepper, sprinkle over half feta.
Remove the inner pot and pour out as much of the cooking liquid as possible. This will expose more of the lamb and veg for crisping.
Swap for the air fryer lid, select Air Fry and set the time for 8 minutes, and temperature to 205°C. Cook until the potatoes and the top of the lamb is a little golden brown.
If you don't have an Instant Pot Duo Crisp or Pro Crisp, tip the lamb and potatoes into a large baking tray, and bake in the oven at 200°C until lightly golden.
Spoon the dish onto a serving platter, add the remaining feta and scatter over a little more picked oregano.