Cut the head of cauliflower into florets and roughly chop the garlic.
Add ingredients into the Instant Pot: garlic, cauliflower florets, cashews, vegetable broth, potatoes, cannellini beans, thyme, rosemary, bay leaf, nutritional yeast, 1.5 teaspoons of sea salt, and black pepper. Stir to combine the mixture.
Close the Instant Pot lid and set the Pressure Release to Sealing. Select the Pressure Cook setting and set it to high pressure for 7 minutes.
Once the 7-minutes have completed and beeps, perform a quick pressure release by switching the Pressure Release knob from Sealing to Venting.
Open the pot and remove any herb sprigs. Use an immersion blender to puree the soup until it's thick and creamy. Alternatively, if you want a super-smooth texture, transfer the soup to a stand blender and blend until smooth (replace the center cap from the blender lid and cover with a dishtowel to allow the steam to escape).
Taste and adjust seasoning accordingly. (I'll add a bit more sea salt at this stage).
Once ready to serve, pour the soup into bowls. Optional: Before serving, zest some lemon over the soup, squeeze some lemon juice over the soup, as well. Add as many red pepper flakes as you like, and drizzle with a bit of olive oil. Serve with salad and bread, if desired.