Drain clams, reserving the liquid. Roughly chop clams and refrigerate until needed.

In a large soup pot, heat avocado oil over medium heat. Add onion, celery, celery root, and cauliflower. Cook, stirring occasionally, until onion is tender and vegetables begin to soften (5–6 minutes).

Add black pepper and salt; cook 1 minute more.

Add clam juice, reserved clam liquid, fish sauce, broth, and coconut milk. Bring to a simmer.

Add bay leaf, thyme, and lemon zest. Reduce the heat to low and simmer for 20–30 minutes, until the cauliflower is very tender and the soup is fragrant.

Remove bay leaf and thyme. Using an immersion blender (or standing blender), blend the soup halfway creamy but still chunky.

Add clams and cook gently over low heat until just warmed through.

Serve topped with minced parsley and chives.