Press on the SAUTE button on the Instant Pot. Heat the oil, and saute the onion until it's soft and translucent. Add the garlic, and cook for a minute or until it's fragrant. Add the diced chicken, season it with salt and pepper and lightly saute it for a few minutes.
If there are bits stuck to the bottom of your pot, deglaze with a splash of water.
Once that's done, switch off the SAUTE setting and add pasta, butter, and ground nutmeg. Then add the vegetable stock.
Give the ingredients a quick stir and try to get all of the pasta to be covered in liquid (add a little bit of water if necessary).
Secure the lid and set the vent to SEALING. Select the PRESSURE COOK/MANUAL setting and set the cooking time for 5 minutes at high pressure. The Instant Pot will take around 10 minutes to come to pressure and start cooking. When the cooking program ends, quickly release the steam.
Carefully open the lid so that the steam doesn’t get in your face. It’s important to quickly release the steam, otherwise, the pasta will be overcooked.
Give the pasta a quick stir then mix in the heavy cream.
Finally, stir in the cheese. Keep stirring until the cheese is completely melted and the sauce is creamy.