Preheat your oven to 400 degrees F. Place your head of garlic into a mold of a regular muffin tin, exposed cloves side up. Drizzle with extra virgin olive oil. Cover with foil and roast for 45-50 minutes or until fragrant, brown and soft. Remove from oven and set aside to cool. While your garlic is roasting, prepare your soup.
In a large dutch oven, heat the 3 tablespoons of extra virgin olive oil over medium heat. Add the onion and celery root. Cook for 10-12 minutes or until the onions are tender. Add the cauliflower, chicken broth, water and 2 teaspoons of salt. Bring to a boil and then lower the heat to a simmer. Cover and cook for 45 minutes or until the veggies are tender.
Now go back to your garlic. Release the cloves from the bulb.
Working in batches, transfer the soup to a blender and add the roasted garlic cloves. Do not fill the entire way to reduce mess. Blend in batches, for about a minute each until completely smooth. Transfer the now pureed soup back to the dutch oven and stir in the heavy cream. Cook for an additional 5 minutes seasoning with salt and pepper where needed.
Serve the soup warm with a drizzle of extra virgin olive oil, chopped chives and ciabatta croutons.