Set the Instant Pot to saute and add the bacon. Fry bacon until nearly crisp.
Add the onion and garlic to the Instant Pot with the bacon and cook until onion has softened and garlic is fragrant.
Add the chicken broth to the pot and stir well, being sure to scrape the bottom of the pan so as not to get a burn error when cooking.
Add the potatoes, salt, and pepper to the pot and stir.
Cover the pot, set the lid to sealing, and turn to high pressure. Cook for 10 minutes.
Release the pressure carefully.
Use a potato masher to mash the potatoes to reach the desired consistency.
Add the flour and milk to a mason jar and close tightly. Shake well to combine. Alternately, whisk the flour and milk together in a small bowl.
Pour flour mixture into the soup and stir to combine. Add the cream and evaporated milk to the soup and stir.
Turn pot to saute and cook, stirring constantly, for 5 minutes or until the soup has thickened to your liking.
Stir the cheddar cheese into the soup until smooth and melted. Taste and add additional salt and pepper, if needed.
Ladle hot soup into bowls and garnish with cheddar, bacon, and green onion, if desired.