Select "saute" on the Instant Pot and add your fat. Add the sliced onion, crushed garlic, salt, and pepper (if using).
Saute onion and garlic for 5-10 minutes, stirring frequently to prevent sticking.
Add broth and cauliflower and continue cooking until cauliflower is soft, about 10-15 minutes.
Press "cancel" on the Instant Pot and transfer contents to a high-powered blender, or use a stick blender to puree until a smooth, creamy soup base forms. Return to Instant Pot.
Add clams (and the juice from the can), sliced carrots, and fish sauce and stir.
Press "soup" button and adjust time to 20 minutes, attach lid, and ensure vent is closed.
Instant Pot will build pressure then begin counting down the time remaining on the display.
Once the soup is done cooking, the Instant Pot will beep. Allow the pot to sit while the pressure slowly releases (10-15 minutes) before removing the lid. The silver indicator next to the vent will go down once it's safe to remove the lid. Alternatively, you can manually release the pressure by opening the vent; again the silver indicator will go down when pressure has been relieved.
Ladle into bowls and top with crispy bacon pieces and chopped green onion. Serve with tortillas or flat bread, if desired.
If you prefer to make this in a crock pot, start by doing steps 2-4 in a medium-sized pot on the stove top before transferring contents to the crock pot. Complete remaining steps in the crock pot and cook for approximately 4 hours, until carrots are soft.
Likewise, if you would like to cook the soup on the stove top, after blending, return contents to a large pot on the stove top, add remaining ingredients, and simmer for approximately 1 hr, until carrots are soft.