Turn the Instant Pot to Sauté. Melt the butter, then cook the onion for 3 minutes. Stir in the garlic and cook 1 minute.
Add the chicken broth, cream of chicken soup, potatoes, salt, and pepper. Secure the lid, set the valve to Sealing, and cook on High Pressure (Manual) for 10 minutes. Carefully perform a quick release.
While the pressure releases, whisk together the milk and flour until smooth. Once the pin drops, remove the lid and stir the mixture into the soup. Turn Sauté back on and cook for about 5 minutes, stirring occasionally, until thickened.
Stir in the shredded cheddar cheese until melted.
Serve warm topped with bacon, green onions, sour cream, and more cheddar cheese if desired.
Notes
Freshly shredded cheddar cheese melts smoother than pre-shredded cheese.
For a thicker soup, mash some of the cooked potatoes with a potato masher or immersion blender before serving.
Taste and adjust the salt and pepper before serving.
Store leftovers in an airtight container in the refrigerator for up to 5 days. Freezing is not recommended.
If the soup becomes too thick after sitting, stir in a splash of milk or chicken broth while reheating.