Slice a piece of cucumber and reserve for garnish. Peel, halve and seed the remaining cucumber. Puree in a blender until smooth, then strain through a fine mesh sieve.
Fill a highball glass three-quarters of the way with ice. Add 2 ounces cucumber puree, the gin, lime juice and simple syrup and stir. (To make a batch of simple syrup, simmer equal parts sugar and water in a saucepan until the sugar dissolves, then cool.) Fill the glass with seltzer and stir again. Garnish with the reserved cucumber slice.
Photograph by Jonathan Kantor
Recipe adapted from Jones (Philadelphia) for Food Network Magazine