Press the Saute button on your Instant Pot to turn on saute mode. Add the butter to the pot. When the Instant Pot is hot, add the onion and cook, stirring often, until onion has softened, about 3 minutes.
Add the garlic to the pot and press Cancel to turn off the Instant Pot. Stir for 30 seconds.
Pour in a little bit of the broth and stir, scraping up any browned bits stuck to the bottom of the Instant Pot. Add the potatoes, carrots, oregano, salt, pepper, cayenne (if using) and remaining broth to the Instant Pot. Stir.
Close the Instant Pot lid and turn the steam release valve to the sealing position. Using the Manual/Pressure Cook function, set the cook time to 6 minutes. The Instant Pot will take about 15 minutes to reach pressure and then the cook time will begin counting down.
When the cook time ends, allow the pot to naturally release for 10 minutes by just letting it sit. After the 10 minute natural release, use the handle of a long spoon to carefully turn the steam release valve to the venting position. Once all of the steam escapes and the pin drops down, carefully open the Instant Pot lid.
In a small bowl or liquid measuring cup, stir together the 1 cup milk and 1 tablespoon cornstarch. Turn the Instant Pot on saute mode and stir in the milk and cornstarch mixture. Use a potato masher to mash some, but not all, of the potatoes. Let the soup simmer, stirring, until it thickens slightly.
Press cancel to turn the Instant Pot off. Serve the soup with toppings, as desired.