Set the Instant Pot to SAUTE and add the butter. Allow the butter to melt.
Add in the chopped onions and garlic, cooking until the onion becomes translucent.
Press CANCEL on the Instant Pot and add in the vegetable broth and carrots.
Add the Dijon mustard, black pepper, hot pepper sauce, nutmeg, and salt.
Add the broccoli florets and close the lid.
Cook on the setting MANUAL/PRESSURE COOK for 8 minutes.
Slowly and carefully perform a quick release (QR) once the time is up.
Carefully remove the lid and add in the cubed cheese and stir to melt. *
Set the Instant Pot back to SAUTE and add in the milk, and cornstarch and water slurry.
Saute for 3-5 minutes and allow the soup to thicken.
Notes
*At this time you can remove half of the soup and puree in a blender or use an immersion blender to pulse the soup in the pot. This step is not necessary.