Remove root, tough outer leaves, and green tops from leeks. Cut leeks in half lengthwise; rinse and drain. Chop leeks.
Cook sherry and 1 tablespoon butter in a large saucepan over medium-high heat until butter melts; add leek, and sauté 5 minutes. Add potato, 4 cups water, bouillon granules, and garlic. Bring to a boil; reduce heat, and cook 15 to 20 minutes or until potatoes are tender. Drain.
Combine potato mixture, 1/4 cup butter, half-and-half, and salt in a large bowl; mash with a potato masher until smooth. .