Turn on your Instant Pot and Select Saute with the option to cook on High.
Once it reads HOT, add the butter and melt. Add onion and cook stirring for 2-3 minutes, until tender.
Add the minced garlic and cook for about 30 seconds.
Add broccoli, carrots, celery, salt, pepper, nutmeg, and red pepper flakes.
Add chicken broth, stir to combine. Turn off Saute mode.
Cover with the lid and lock, make sure the vent is set to seal. Select High Pressure and set the timer to cook for 2 minutes.
When the cooking time ends, do a 5 mins Natural Pressure Release, and release manually the remaining pressure by moving the valve from seal to vent. When the valve drops, remove the lid carefully.
In a bowl whisk together cornstarch and cream until fully combined.
Select Saute mode again and slowly add the cream mixture. Stir to combine.
Turn off the pressure cooker and use an immersion blender to puree about half of the mixture, leaving some veggies intact.
Slowly add the cheese a handful at a time and stir well until it melts. If the soup is too thick, add more broth or cream and simmer.
Adjust salt and pepper to taste and serve hot with your favorite toppings.