The Green Lantern

-In a blender, puree 1 cup flat leaf parsley leaves, the zest from 2 limes, ½ cup fresh limejuice, 1 cup cold water and ½ cup sugar for 1 minute. 
-Strain through a fine-mesh sieve into a large glass-measuring cup. 
-Add ice to 6 glasses and divide mixture between glasses. 
-Add a shot of gin or vodka if you wish, and top with club soda, a lime slice and a small sprig of parsley.