Turn the Instant Pot to saute mode and add the butter.
Saute the onions, potatoes, thyme, worcestershire sauce, and mushrooms just until the onions are soft. About 5 minutes.
Add the broth.
Close the Instant Pot and set to pressure cook for 5 minutes.
When finished, use the manual release to release the pressure.
In a small bowl, mix the flour and ½ and ½ to create a slurry
Turn the Instant Pot on to saute and bring the soup to a low boil (it will already be hot so this shouldn't take long).
Slowly pour the slurry into the soup and whisk well. Let it heat and thicken. Add salt & pepper, as desired.
Serve immediately or blend and store.