Slice the celery into 1/4 inch pieces. Roughly chop up the onion. Chop the carrots into 1/2 inch pieces. You can peel the carrots if you want to, but don’t have to as long as they’re washed well. Slice the mushrooms.
Place the stainless steel pot in the instant pot and turn on “Saute”. Add the butter to the Instant Pot stainless steel pot and allow to melt. Then add the celery, carrots, onion, and mushrooms in that order.
Add in the wild rice and vegetable broth.
Seal the lid and close the steam release valve. Press the "Pressure Cooker" button until it's on "Normal", press the "Pressure Level" button until it's on "High" and then press the +/- buttons until it says "00:20” (for 20 minutes).
While the Instant Pot is cooking combine the cornstarch, thyme, and water in a small bowl.
When the Instant Pot is done, press the “Cancel” button to turn it off and quick release the pressure by opening the steam release valve, staying away from the steam. If it seems to be releasing liquid, not just steam, then allow the Instant Pot to sit for a minute or two before beginning to release pressure again.
Turn the Instant Pot on “Saute” and add the cornstarch mixture. Bring to a boil.
Then turn off the heat by pressing “Cancel” and add the sour cream. Mix the sour cream and add salt to taste. Serve.