Place potato chunks into inner pot of pressure cooker.Add celery, onion, garlic, seasoning salt, pepper and butter. Stir to combine.Add bacon and chicken stock to pot and then stir to combine. (I debated on whether to add the bacon, now, or when the soup was done. Now, meant sinking that smoky flavor deep into the potatoes. Later, meant having that crispier texture element to enjoy in my finished soup. Tonight, I opted for the deep flavor (Totally delish, by the way!), but you could go either way and it would be tasty and perfectly acceptable.) If you decide to wait, you will add the bacon when you stir in the milk and cream.)Place the lid on the Instant Pot slow cooker and lock it into place.It will start, off to the side, so it drops into the slots in the rim and then you will grip the handle to spin the lid into the locked position. See how the arrows line up when it's locked?Push the Manual mode button and set the timer for 5 minutes. (I started with 10 and then checked half way through. At five minutes, the potatoes were fork tender and perfect. Amazing!)Quick-release the steam from the pressure cooker.When steam is fully gone, remove the lid of the slow cooker. There should still be liquid in the pot with your veggies.Use a potato masher to crush vegetables, resulting in a thick, semi-smooth mash. I left a few bigger chunks of potato, in mine, for texture. That's totally a preference thing, so adjust for what your family likes.)Add cream and whole milk, stirring to incorporate.Serve hot and steaming, topped with sour cream, shredded cheddar and sliced green onion.