Make the wet masala: put the second onion (chopped roughly), and ginger and garlic (peeled and chopped) in a blender along with half a cup water and whiz to get a smooth puree.
Chop the tomato.
Peel the potatoes - optional if using baby potatoes
Cooking
Turn the instant pot to saute (normal) and add the sliced onions. Fry them until reddish brown.
Add the wet masala, tomatoes, and ground spices. Cook for 10 minutes or until the tomatoes break down.
Add the lamb and salt and cook for 5 minutes more, stirring the contents with a spoon every now and then. Add a few splashes of water if you see the masala getting dry.
Add 2 cups water and cancel sauté.
Secure the lid, press the pressure cooking button (high), set the timer to 15 minutes and make sure the steam release handle is sealed.
Once the timer beeps, wait for 10 minutes, then release steam by turning the steam release handle to venting.
Open the cooker and add the potatoes. Again, secure the lid and set the cooker to one minute pressure.
When the timer beeps, open the cooker and add green chillies and cilantro. Serve aloo gosht with warm naan.
Notes
You can substitute lamb with mutton or goat meat.
Use bone-in meat as the bones add a lot of depth and flavour. Lamb shoulder has the best and most succulent meat.
If you don't have ground garam masala, you can use whole spices too - 2 black cardamom, 3 green cardamom, 2 bay leaves, ¼ teaspoon black peppercorns and 4-5 cloves will be enough.
Russet potatoes work well in this recipe too. If they're big ones, peel and halve or quarter them depending on their size. You'll want medium chunks of potato, not small pieces.
Depending upon the type and size of potatoes, sometimes it takes more time for them to soften. I typically find that peeled and halved potatoes get done in 1 minute of pressure cooking, but you might get different results. If the potatoes are not done in 1 minute, just put the lid back on and pressure cook for another minute.