Remove 8 thin strips of zest from the lemon using a vegetable peeler and set aside. Thinly slice the lemon into rounds and put into a small saucepan along with the sugar and 1/3 cup water. Bring to a boil and cook, stirring, until the sugar dissolves. Let cool at room temperature and strain.
Rub a strip of zest around the rim of 8 martini glasses, and then drop into the bottom of each glass for garnish. Combine the gin, elderflower liqueur, lemon syrup and ice in a measuring cup or pitcher and stir until very cold. Strain into the prepared glasses.
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