Set the Instant Pot to saute and add the butter. When the butter is foamy, add the onions and saute them until they start turning golden brown. Add the garlic and saute another 30 seconds. When the garlic is fragrant, add the cauliflower and saute for about 5 more minutes, turning occasionally, until the cauliflower begins to brown.
Add a cup of broth to the Instant Pot, put on the lid, and set to cook on high pressure for 3 minutes. Use the manual release, then open the cooker when it's safe to do so. Add the rest of the broth and use an immersion blender to puree the cauliflower.
When the cauliflower gets smooth, stir in the cheese, heavy cream, and sour cream. Add the salt and pepper, then use the immersion blender to puree it and mix everything together. Serve in a small bowl with your desired toppings.
Macros for just the soup: 209 calories, 8 grams net carbs, 7 grams protein, 16 grams fat.