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