Heat a large stock pot over medium heat and add 2 Tbsp. of a high-heat cooking oil. Add the chopped onion, leek, celery, and sea salt and stir. Continue to cook until the vegetables have turned translucent, about 5-10 minutes. Add the garlic and stir for 60 seconds. Add the chopped cauliflower and broth and stir well. Bring to a simmer. Cover and cook for 15-20 minutes, or until the cauliflower is very soft.
Add half the soup to a blender with 1 Tbsp. butter (the blender should only be about half full to avoid too much pressure from the hot liquid; blend with caution). Blend on high speed until smooth and creamy, about 60 seconds. Pour the soup out into a large serving bowl and repeat with the second half of soup and remaining 1 Tbsp. butter. Taste the soup and season with sea salt and black pepper, as desired.