Heat oven to 400 deg F. In alarge roasting pan, toss cauliflower, onion slices and garlic with olive oil.
Roast at 400 deg F for 30 mins stirring half way through.
In large saucepan, combine roasted cauliflower mixture, chicken broth, 1 cup of water, bay leaf and thyme. Cover; bring to boil. Reduce heat; simmer covered for 20 mins.
Discard bay leaf. In blender or food processor, puree soup in batches. Return to pan stir in half and half, salt and pepper; cook over medium heat until heated through.