Set the Instant Pot to Sauté and add in the butter and the onion. Sauté until onion has softened, about 5 minutes.
Add in the carrot, celery and garlic. Stir.
Add in the nutmeg, thyme, cayenne and 1/2 tsp each salt and pepper.
Add in the broccoli, cauliflower and vegetable broth.
Set Instant Pot to high pressure for 8 minutes.
Quick release when done and use an immersion blender to partially blend the soup (I like to leave it chunky)
Set the Instant Pot to sauté again. Whisk the cornstarch into the milk and add to the pot along with the cream. Let simmer until thickened and then stir in the cheddar until melted. Add more S&P to taste.
Keyword broccoli cheddar soup instant pot, cauliflower soup instant pot