Break tips from beans; remove strings, if necessary. Wash in cold water, and drain. Place beans with water to cover in a medium saucepan. Bring to a boil and cook 6 minutes; drain well. Cool completely.
Combine dried whole tarragon, olive oil, vinegar, garlic, salt, mustard, and pepper; mix well, and pour over beans. Cover and refrigerate overnight, tossing occasionally. Transfer to a serving dish; garnish with tarragon leaves.