Set the Instant Pot on SAUTE function and keep it at the NORMAL heat setting.
Add butter and when the butter melts add the onion and saute until they begin to turn limp.
Reduce the heat to LOW settings and add all purpose flour. Saute the flour stirring briskly until the raw smell disappears. About 30 seconds. Do not stop stirring otherwise the flour will burn and you've to start all over again.
Pour the chicken stock and de-glaze the pot well with the help of a spatula scraping the sides and the bottom of the pot to loosen any stuck up bits or crumbs. This will also help dissolve the roux uniformly in the liquid and prevent the BURN error!
Now, add the peeled, diced and rinsed potato cubes. Also, add crushed black pepper according to your preference, Bring the ingredients into a boil. CANCEL the SAUTE function.
Close the lid, seal the valve and Press the PRESSURE COOK function. PRESSURE COOK on HIGH for 3 minutes.
Once, the cooking cycle is over wait for Natural Pressure Release (NPR) for 10 mins. Than, release the rest of the pressure manually by moving the valve from the SEALING to the VENTING position. When the floating pin drops open the lid carefully.
Turn on the SAUTE function and set it on LOW heat setting. Now, add warm milk and more crushed black pepper as per your liking and give everything a quick stir. Bring the soup to a boil first and than simmer for a few mins. Stir often.
Next, grab a potato masher and mash the cooked potatoes roughly inside the pot itself. This will give you a creamier yet slightly chunky soup. However, if you prefer super chunky soup mash only a few potatoes roughly to add body and texture to the soup.
Add the sour cream, heavy cream, crispy fried bacon, chopped scallions and sharp cheddar cheese and mix well with the soup. If you prefer super thick soup you may simmer the soup for a few minutes on LOW settings (SAUTE function) or add 1 tbsp cornstarch + 1 tbsp water (made into a slurry) to thicken it up quickly.
Turn off the Instant Pot. Dish out the soup topping with more sharp cheddar, scallions, fried bacon bites & dollop of sour cream. Dig in!