Soak the cashews in enough water at least for about 15 minutes in advance.
Slice the onions finely, keep aside.
Heat oil in the instant pot (press saute function and adjust to low), add the cumin seeds.
As the cumin seeds are lightly fried, add the chopped onions and enough salt, saute for about 6 to 7 minutes until the onions are lightly browned. As needed you may adjust the heat. You can do this by pressing the cancel button and then press the saute button again and use the adjust button to use low or normal settings as needed. Make sure to stir in between so the onions won't stick to the bottom of the pot.
As the onions are sauteed enough, add the ginger garlic paste, mix well and saute for another couple of minutes.
Transfer the soaked cashews to a blender (discard the water used for soaking) along with fresh spinach, green chili and 2 cups of water, blend the mixture until very smooth.
Transfer the blended spinach cashew mixture to the pot, stir well with the onions and add the butter and all the spice powders, mix well and saute the contents for about 1 minute.
Add the remaining 1 cup of water and enough salt, mix well.
Close the instant pot with lid in sealing position and press manual, cook in high pressure for 6 minutes of time.The instant pot will take few minutes to build up pressure and start cooking. Once the cooking is done, the pot will beep so you will have to wait until then.
Once the cooking is completed and the pot beeps, press the 'keep warm/cancel' button and wait for a natural pressure release of about 15 minutes.
After an NPR (natural pressure release) of 15 minutes, carefully do a manual release by turning the lid to the venting position slowly and carefully, releasing the remaining pressure gradually.
Once the pressure is all released, carefully open the lid, mix the contents.
Add the sliced paneer cubes, mix well and add little more water if needed for gravy (I added about 1/2 cup of water for gravy).
Press the saute function, adjust to low and simmer the contents for 2 to 3 minutes until the paneer is cooked and soft. If you simmer it for long, the paneer slices may crumble so a couple of minutes will be enough and as soon as the paneer is cooked, press the cancel button to turn off the saute function.Add the heavy cream (if using), gently mix in the cream and serve the delicious palak paneer curry hot with chapathi, rice or any flatbread as you like.