Place the kiwi pieces in a cocktail shaker and muddle until they release their juice.
Add gin, elderflower liqueur, lemon juice, and simple syrup to the shaker.
Fill the shaker with ice and shake for 15-20 seconds.
Strain the mixture into a Collins glass filled with fresh ice.
Top with club soda and stir gently with a bar spoon.
Garnish with a thin kiwi slice.
Notes
The drink works best with ripe kiwis that are soft but not mushy.Store-bought simple syrup works fine, or make your own by mixing equal parts sugar and hot water until dissolved.If the drink tastes too sweet, add an extra 1/4 oz of lemon juice.The cocktail can be made non-alcoholic by replacing gin with cucumber-flavored sparkling water and elderflower liqueur with elderflower syrup