Peel potato and dice fine. In a 3 1/2- to 4-quart saucepan cook garlic and onion in butter over moderately low heat, stirring, until onion is softened. Add potato and leeks and cook, stirring occasionally, 2 minutes. Stir in broth and simmer, uncovered, 15 minutes, or until potato is very tender. If desired, in a blender purée soup in batches until very smooth (use caution when blending hot liquids), transferring to another saucepan. Stir in cream, parsley, and salt and pepper to taste and heat over moderate heat, stirring occasionally, until hot.