Make cucumber juice: Blend 1 large cucumber, then strain through a fine mesh sieve or cheesecloth to yield about ½ cup juice.
In your pitcher, stir together cucumber juice, lemon juice, syrup, elderflower liqueur, and gin.
Chill until ready to serve.
Add ice and soda water to the pitcher just before serving, or pour the base mix into ice-filled glasses and top each with ~⅓–½ cup soda water for max fizz.
