Step 1

Using a vegetable peeler, shave 4 long, thin ribbons from cucumber; set aside. Cut a 4" piece from remaining cucumber and coarsely chop. Muddle with kaffir lime leaves, sugar, and 2 lemon slices in a cocktail shaker. Add gin and lime juice and fill shaker with ice. Cover and shake until outside of shaker is frosty, about 20 seconds.

Step 2

Strain into 2 ice-filled Collins glasses. Top off each with club soda. Gently stir to combine; garnish each with 2 reserved cucumber ribbons and remaining lemon slices.