In a soup pot, melt the butter. Add the onion, shallot and garlic and cook over moderately low heat, stirring occasionally, until softened, about 5 minutes. Add the cauliflower, thyme and bay leaf and cook, stirring occasionally, until the cauliflower is barely softened, about 5 minutes. Add the broth and water and season with salt and pepper. Simmer over moderately low heat until the cauliflower is softened, about 30 minutes. Discard the thyme sprigs and bay leaf.