Scrape the zest from 2 lemons (about 1/8 tsp) and set aside. Cut the lemons in half then extract the juice.
Preheat the Instant Pot on Sauté-high until hot. Add the oil/butter. Stir in the onion, garlic, basil, and parsely. Sauté until lightly browned.
Stir in the chopped cauliflower. Add the broth, salt and pepper to taste, the lemon zest, and half of the lemon juice.
Cancel the Sauté mode and cover the pot. Reset to Pressure Cook, high, 5 minutes.
De-pressurize right away. Add the rest of the lemon juice then use the immersion blender to smooth the soup. Adjust salt and pepper to taste. Serve with a garnish of sliced lemon and chopped parsely.