Start by roughly chopping the onion. Cut the red potatoes into medium chunks - for smaller potatoes you can cut them in half and for larger potatoes cut them in quarters. Cut the celery into thin slices. Cut the kernels off of the corn if using fresh corn.
Place the stainless steel pot in the Instant Pot. Press the “Saute” button until it’s on “Normal”. Add the butter.
When the butter is melted, add the onion and saute for 2 minutes, stirring occasionally. Then add the celery and saute for another 2 minutes. Make sure to scrape up any burned bits on the bottom.
Reserve 1/4 cup of the broth in a small bowl and add the rest to the Instant Pot. Also add in the potato chunks and the fresh or frozen corn.
Lock the Instant Pot lid in place. Close the pressure release valve and press the “Pressure Cook” button until it’s on “Normal” and press the “Pressure Level” button until it’s on “High” (“High” is the default for the Duo model). Press the +/- buttons until it’s on 00:05 for 5 minutes.
The Instant Pot will say “On” while it’s coming up to pressure and then will start counting down. When it’s done, allow the pressure to release naturally for 10 minutes by just leaving the Instant Pot alone and allow the time to count up until it says L00:10.
Quick release the remaining pressure by opening the pressure release valve. When the float valve is down, remove the lid.
Turn the Instant Pot back on “Saute”.
Add the cornstarch to the reserved broth and combine well. Add the mixture to the Instant Pot along with the salt and the dried thyme. Mix together for about 2 to 3 minutes until the soup has thickened slightly.
You can make the soup completely smooth and creamy, or go somewhere in between (my favorite!). To make the soup creamier, blend with an immersion blender until it’s as creamy as desired.
Finally, add the whole milk and heavy cream and mix together. Then serve!