Turn the Instant Pot to the Saute function, and add 3 tbsp of the butter. When melted and bubbling, add the onions, ginger, and garlic. Saute until the onions are soft and translucent (2-5 minutes).
Add the spices, reserving 1 tsp of the garam masala for later, and cook while stirring for 30 seconds. Add the water to deglaze the pan, and thoroughly scrape the bottom. Turn off the Saute function.
Stir the onion mixture well. Dump the can of tomatoes on top, but do not stir. Place the chicken thighs on top of the tomatoes.
Put the lid on the Instant Pot, turn the knob to Sealing, and set to High Pressure for 10 minutes,
When the time is up, allow to naturally pressure release for 10 minutes before venting. Take off the lid and remove the chicken, set aside.
Blend the sauce with an immersion blender (or transfer to blender) until smooth. Turn on the Saute function and cook until until thickened slightly (2-5 minutes). Add the butter, remaining garam malasa, and heavy cream. Stir until the butter melts, and everything is incorporated.
Cut the chicken into pieces, and return to the sauce. Mix through. Serve with Basmati rice, and garnish with cilantro.
Basmati Rice
Combing equal parts rinsed basmati rice and water in the Instant Pot.
Put the lid on, turn the knob to Sealing, and set to High Pressure for 6 minutes.
When time is complete, allow to naturally pressure release for 10 minutes before venting. Remove the lid and fluff the rice.