Combine the water or stock, potatoes, leeks, garlic, and salt and pepper to taste in a saucepan, cover, and turn the heat to high. Bring to a boil, then lower the heat so the mixture simmers steadily but not violently. Cook until the potatoes are tender, 20 to 30 minutes. Cool or chill, then season to taste.
Puree in a blender, then chill fully. Stir in the cream, then adjust the seasoning and serve, garnished with parsley.
