Combine sugar, vanilla bean, 3 small fresh rosemary sprigs, and water in a medium saucepan. Bring to a light boil over medium heat, stirring occasionally. Simmer 5 minutes. Remove from heat, and let cool 30 minutes. Pour through a fine wire-mesh strainer into a large pitcher, discarding solids. Stir in fresh lemon juice. Cover and chill 3 to 48 hours. Stir just before serving over ice. Garnish, if desired.
* 3 (5-oz.) containers frozen lemon juice, thawed, may be substituted.