Set your instant pot to sauté mode. Once hot, add in your butter. Add your onion and sauté for 3 to 4 minutes until it begins to soften.
Once your onion has softened, add in your garlic. Sauté for about a minute to release the flavors of the garlic.
Press cancel on your Instant Pot. Add in your potatoes, thyme, nutmeg, broth, salt, and pepper.
Close and seal the lid on your Instant Pot. Close the valve. Select pressure cook on manual high for 4 minutes. Once the time is done cooking, naturally release for 4 to 5 minutes, then do a quick release by opening the valve.
Press cancel on your Instant Pot, then sauté mode. In a separate bowl, mix your milk and corn starch. Once completely mixed, add to your Instant Pot. Carefully stir the corn starch mixture making sure not to mash any potatoes to keep it second chunky. Allow it to sauté for about three minutes to thicken up.
Mix your sour cream. Once fully incorporated, press cancel on your Instant Pot.
Garnish your potato soup with classic toppings such as bacon, cheese, and green onions!
Plate, serve, and enjoy!