Sauté onion, celery, carrot, and minced garlic over low heat with vegan butter until onions are soft and almost clear (approx 5-7 minutes.)
Then add cauliflower pieces, vegetable broth, nutritional yeast flakes, and seasonings.
Bring to a simmer over medium heat, cooking until cauliflower is just fork-tender (avoid over-cooking).
Add cashews, yeast flakes, and two cups of cauliflower soup mixture to blender and process carefully until completely smooth. Pour into a large bowl and continue processing batches of soup mixture and transferring into the bowl until everything is pureed.
Return creamed soup to a pot and keep warm until serving. Garnish with chopped chives or croutons as desired.