Turn your Instant Pot on to saute and add butter. When Butter is melted, add garlic, green peppers, and onions. Cook until the onions start to become translucent (about 3-5 minutes).
Add mushrooms and cook for another minute. Click the off button on the Instant Pot.
Add roast beef, beef stock, and seasonings.
Place lid on Instant Pot and cook on manual high pressure for 10 minutes. When timer is up, allow for a 10 minute natural pressure release. After 10 minutes, release all of the remaining pressure.
Turn Instant Pot on to saute and stir in the milk/flour mixture. Cook on saute until soup starts to thicken (about 3-5 minutes). When soup is thick, add cheese and stir until well combined.