Cut the core out of the cauliflower head. Break the rest into small florets, trimming off any large stems. You should have about 89 cups. Bring several inches of water to a boil with a steamer basket insert in a large pot. Add cauliflower, cover, and steam 13-15minutes or until very soft. Drain water, and place cauliflower back in the pan to stay hot.
Trim off tough green top and root end of the leek. Use the white and pale green part only. Cut leek in half lengthwise, and run under cold water to remove sand and dirt. Cut the leek halves crosswise into thin slices.
Heat 1 tablespoon of the olive oil in a large sauté orfry pan over medium heat. Slowly cook leek with thyme over medium low to medium heat until soft. Do not brown. Add garlic, and cook 1 more minute. Sprinkle with salt and pepper.
Add hot cauliflower and leek mixture to the bowl of a food processor. Add the Parmesan, the remaining 2 tablespoons olive oil, and sour cream. Puree until smooth and creamy. Taste and add salt and pepper as needed.