Melt the butter in your Instant Pot set on the sauté setting. Add the onions and cook, stirring, until the onions are softened, about 5 minutes. Add the garlic and cook for an additional minute.
Add the cauliflower, chicken stock, and salt to the pot. Close the lid and position the steam release handling to "Sealing". Select "Manual" "Normal", and adjust the time to 7 minutes. The pressure cooker will then enter the preheating cycle after 10 seconds. Mine took about 10 minutes to preheat. Once it comes to pressure, the timer will start counting down from the selected 7 minutes.
When the timer sounds, release the pressure using the quick release method. Check the cauliflower to make sure it's soft. Purée the soup using an immersion blender, or in batches in a blender, then pour back into the pot.
Add the cream cheese and cheddar cheese and blend again to melt the cheeses. Serve hot with additional cheese and desired toppings.
Suggested toppings include additional cheese, crumbled bacon, chopped scallions.