With your Instant Pot on the saute setting, heat your oil.  When it's hot and shimmery in the bottom of the pot, add the onions and saute until softened (about 3 minutes if you've diced them small, or a minute or so if you've done them in a food processor).  Add in the garlic and saute for 30 seconds, then turn off the saute setting.

Add in the beans, chicken broth, chili powder, cumin, pepper, 1 tsp salt and green chilies. Stir to combine.

Add the chicken breasts to the pot.  Cover the Instant Pot and secure the lid, making sure valve is sealed.  Set the manual button or pressure cook button to 20 minutes.  Turn off the pot after 20 minutes, letting the pot naturally release for 10-15 minutes. Release any remaining pressure.

Remove the chicken and place on a cutting board. Shred it and return it to the pot.

Add the flour to the warm milk and whisk together until you've made a smooth slurry.  Return the pot to the saute setting and drop in the butter, stirring until it has melted into the chili.  Gradually stir in the milk and flour slurry, letting mixture thicken the chili, about 5 minutes. 

Add sour cream to the chili, and stir until it has melted into the mixture and is completely combined.  Season with more salt and pepper to taste.  Serve with red pepper flakes and grated cheese, if desired.