Start by preparing the simple syrup. Mix equal parts water and sugar (I used granulated) in a saucepan over low heat until the sugar is dissolved. Leave aside to cool.
In a cocktail shaker muddle together a handful of rosemary leaves, grapefruit juice, and simple syrup.
Add gin to the shaker and fill with ice. Shake for roughly 30 seconds, until shaker exterior is frosted and contents well blended.
Pour into 2 ice filled glasses. Top with prosecco and rose lemonade.
Optionally garnish with grapefruit slices and rosemary sprigs.
