Preheat oven to 400 degrees.
In a large roasting pan combine cauliflower, onion, and garlic. Drizzle with oil, toss gently to coat. Spread veggies in an even layer. Roast, uncovered, for 30 minutes, stirring once.
In a 4 quart dutch oven combine roasted veggies, broth, water, bay leaf. Bring to boiling, reduce heat. Simmer, covered, for 20 minutes, cool slightly. Remove and discard the bay leaf.
Transfer vegetable mixture in batches in a blender. Cover and process or blend until smooth. Return pureed mixture in saucepan. Stir in half and half, salt, and pepper. Heat, but not boil.