In a 3 1/2- to 4-quart saucepan cook onion, garlic, and curry powder in butter over moderately low heat, stirring, until onion is softened.
Peel and core apple. Chop apple coarse and add to curry mixture. Add cauliflower, broth, and water and simmer, covered, until cauliflower is very tender, 15 to 20 minutes.
In a blender or food processor purée soup in batches until very smooth, transferring as puréed to another saucepan. Stir in cream and salt and pepper to taste and heat over moderate heat until hot.
