In the bowl of your instant pot, add the potatoes, water, salt, garlic, and sage. Note: you should add enough water to just cover the potatoes but never add water over the fill line.
Attach the lid and ensure the vent is in the “sealing” position. Cook the potatoes on Manual— Normal for 15 minutes. Note: The instant pot will take 5 to 10 minutes to come to pressure and display “on” on the screen. Once at pressure, it will begin counting down from 15.
Once the instant pot has finished cooking, manually release the pressure by moving the vent to the “venting” position. Note: It will steam and may splatter for the first minute. I recommend moving the valve with a pair of long tongs.
Once the pressure has released, drain the potatoes and return them to the instant pot (or a large bowl).
Add the remaining ingredients (sour cream, parmesan, heavy cream, butter, salt, sage, and garlic powder) and mash until the potatoes are your desired consistency. See notes.
Mashed potatoes can be kept warm using the “Keep Warm” setting on the Instant Pot for up to one hour. If they begin to thicken too much, add a ¼-½ cup of heavy cream and stir it to combine. This will help moisten and loosen the potatoes again.