Add 2 TBS butter, onions, garlic, potatoes, and chicken broth to the Insta Pot in that order. Cook on high pressure for 3 minutes then do a quick release of the pressure.
Meanwhile, cook bacon in a large frying pan and chop up, grate cheese, and chop the green onions
When potato mixture has finished pressure cooking, do the quick release then open the lid when done. Turn on the saute mode and mash potatoes with a spoon or masher, leaving some chunks. Stir in butter, salt + pepper, and flour, whisking to combine.
Stir in milk and sour cream then bring just to a simmer. Turn off saute mode and then ladle into bowls, topping with bacon, cheddar cheese, green onions, and sour cream.
Notes:
Leftover soup can be frozen up to a month in glass containers!