Press the saute function on your Instant Pot
When hot, add the butter. Once melted, add the mushrooms, onions, garlic, Dijon mustard, rosemary, Worcestershire, salt, and pepper. Stir, and cook until the mushrooms and onions cook down (about 3 minutes). I like to keep a few of the sauted mushrooms out of the soup to add after blending.
Hit cancel and scrape the brown bits off the bottom of the insert. If you don't do this, you could get the BURN signal.
Add the broth.
Put the lid on, set to seal, and set to high pressure for 7 minutes.
When the 7 minutes up, carefully quick release the pressure.
Using an immersion blender, blend soup to desired consistency. Do not put hot liquid in a blender with the lid on.
Combine the flour and water in a small bowl to make a slurry. Add it to the pot.
Add the optional cream and stir until all warm.