Add butter to the Instant Pot and turn on to Sauté. Select Normal and set timer for 8 minutes.
Once butter has melted, stir in garlic, shallots, onion and celery. Cook until vegetables are softened, stirring occasionally, about 3–5 minutes.
Add thyme, salt and pepper and sauté another 3 minutes.
Turn the Instant Pot off. Add clam juice and cauliflower florets to the Instant Pot and place the cover on. Make sure the steam release handle is on correctly. Turn the Instant Pot on to Pressure Cook and select High. Set to 10 minutes.
Once the Instant Pot timer goes off, let the pressure out by turning the steam release handle. The float valve will drop when the pot is depressurized. Remove the lid.
Using an immersion blender, pulse the chowder a few times so that it is somewhat smooth but still has some chunks of cauliflower. Add the cream and drained clams and stir until combined. Ladle the chowder into bowls and top with fresh parsley.