Add oil to the Instant Pot and turn on the sauté button. Once it displays HOT, add in the onions, sprinkle some salt and cook for 3-4 minutes until soft. Add in the garlic and cook for one additional minute until fragrant, stirring constantly.
Add the broth, ground black pepper, seasoning and potatoes to the pot, and stir well. Deglaze the pot. Cancel SAUTE.
Close the lid on the Instant Pot and turn the vent valve to "sealing." Cook over manual high pressure for 6 minutes. After it's done cooking, a quick release the pressure after 5 minutes.
Whisk the flour and milk together and make a slurry. Once the pin drops, open the lid and stir in the milk and flour slurry.
Turn the pot to sauté and cook the soup for 4-5 minutes, until thickened. Using the back of your spatula, roughly mash some of the potatoes. You can also use potato masher. Creamy potato soup is ready !
Ladle the soup in serving bowls. Serve garnished with chopped chives or green onions, cheese, red pepper flakes. Enjoy!