Add the kumquats, rosemary and sugar to a cocktail shaker.
Muddle until the juices release from the citrus and the sugar dissolves.
Add the gin and ice then close the shaker and shake vigorously.
Pour into a cocktail glass filled with ice and top with club soda and a sprig of rosemary.
Notes
Muddle the kumquats, rosemary, and sugar until the fruit releases its juices and the sugar dissolves. This helps balance the kumquats' tartness while releasing the rosemary's aromatic oils. Shake the mixture thoroughly with gin and ice, then add the club soda after pouring so the cocktail stays fizzy.