1.    In a medium-sized saucepan, combine blueberries, lemon zest and 1 cup water. Over high heat, bring to a boil; reduce heat and simmer until blueberries are very soft. Set a fine strainer over bowl; strain mixture, pressing with the back of a large spoon. Stir lemon juice into purée; chill.
2.    In a saucepan, combine sugar and 1 cup water; over high heat, bring to a boil; reduce heat and simmer until sugar dissolves, 2 to 3 minutes. Transfer to a bowl; chill.
3.    Stir sugar syrup into blueberry purée. Pour mixture into a shallow pan; freeze blueberry mixture until ice crystals form around edges of pan, 45 to 60 minutes. With a fork, scrape the ice crystals from edges and stir into mixture. Freeze mixture until fully frozen, about 1 to 1-1/2 hours, stirring several times. 
4.    To plate: Serve in martini glasses. Garnish with crisp cookies, such as langues du chat, and fresh blueberries, if desired.

Yield: 1 quart