Preheat oven to 400°.
Clean and cut the cauliflower into medium segments. Clean and cut parsnips into quarters. Cut ½ large onion into quarters. Using a sharp knife, cut 1/4 to a 1/2 inch from the top of the two garlic heads, exposing the individual cloves of garlic.
Place vegetables, onion and garlic in a 9 X 13 non stick baking pan. Drizzle all items with approximately ¼ cup of olive oil. Sprinkle vegetables with two teaspoons of Za'atar and one teaspoon of Kosher salt.
Roast vegetables, onions and garlic for 30 minutes in a 400° oven.
Once the vegetables have been roasted, remove from the oven. Combine roasted vegetables and 32 oz of chicken stock in three quart sauce pan. One garlic head should be gently squeezed to add all of the individual garlic cloves to the soup mixture. Place all contents on medium heat.
Carefully use an immersion blender to purée the soup. If you prefer a bit of texture, you can coarsely purée the soup.
At this point, I like to sample the soup for taste. I like to add two more teaspoons of Za'atar and additional kosher salt. I also like to add the additional garlic head in this soup and blend all ingredients with the immersion blender. Roasted garlic is much more mild than raw garlic which is why I like to add the additional garlic head. However, use your taste buds to guide you in the direction that best suits you.
The texture of the cauliflower and parsnips alone makes this soup lovely and creamy. However, ½ cup of whole milk or heavy cream can add another layer of creaminess to the soup. For a vegetarian soup, substitute the chicken stock for vegetable stock.
Once the soup is fully blended, simmer on medium heat for 5 minutes. Serve the soup in bowls and lightly sprinkle with Za'atar, a drizzle of olive oil or a teaspoon of Harissa paste.