Set Instant Pot to sauté mode and sauté onions with butter/olive oil, salt and black pepper for 5-7 minutes (stirring frequently so they onions don’t stick to the bottom).
Then add garlic and sauté for 1 minute more.
Turn off Instant Pot.
Add to Instant Pot, the broth, potatoes, cauliflower, broccoli, and crushed red peppers (if adding) and stir.
Place lid on the Instant Pot, set to sealing, and set Instant Pot to manual for 15 minutes.
When timer is up, let the Instant Pot naturally release for 10 minutes and then quick release.
Puree with an immersion blender or transfer to a blender and puree.
Portion soup into bowls, and top with scallions if desired.