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