Preheat the oven to 350F. Place the slices of cauliflower on a dry cookie sheet and roast until tender, about 15-20 minutes.
To make the chimichurri, place the last seven ingredients in a blender and process until finely chopped. Set aside.
In a large heavy pot, heat the butter and oil until hot. Saute the onion, leek, garlic, and celery in the butter oil mixture until soft and very aromatic. Add the salt. Add the roasted cauliflower and toss with a large spoon until the cauliflower is completely covered with the onion mixture. Cook for another 2 minutes.
Add the broth and water. Bring to a boil and then simmer for 5 minutes. Puree with a emulsion blender until fairly smooth, or less if you prefer it chunky. Pour back into the pot and reheat to a simmer. Add the parmesan cheese and cream.
Stir in the chimichurri sauce until well incorporated. Serve hot in soup bowls. Drizzle some chimichurri sauce over the soup.