Marinade the lamb pieces with yogurt and turmeric powder, and let it sit for at least half an hour.
Set the Instant-Pot to saute mode, and add in the oil to heat. Add in the cumin seeds, cinnamon stick, black cardamom and whole peppercorns and let them start to sizzle.
Add in and saute the onions, ginger and garlic, stirring occasionally, till they begin to soften and turn slightly brown. Mix in the spices, salt and tomato paste, and fry for a minute or two to blend well.
Slowly stir in the lamb with all the leftover marinade and add in enough water to cover the meat. Put on the lid and set the Instant-Pot to Manual Pressure mode, and make sure that the valve is in the sealing position. Set the pressure timer to 20 minutes and let it cook.
Once the timer goes off, release the pressure manually, and carefully remove the lid. Add in the potatoes and stir the curry to mix well. You can add in a bit more water depending on the consistency of the curry you prefer. Close the lid and set the Instant-Pot to Manual Pressure mode for 5 minutes, making sure that the valve is in the sealing position.
Once the timer goes off, let the pressure release naturally, then give it a final stir and garnish with fresh cilantro leaves. Serve hot.