Fill a cocktail shaker with ice. Add gin, limoncello, lemon juice, and rosemary syrup.
Shake for about 15 seconds until cold. Strain into a chilled cocktail glass with fresh ice.
Top with club soda or sparkling water. Stir gently.
Garnish with a rosemary sprig and a lemon slice.
Notes
Make rosemary syrup by simmering equal parts sugar and water with rosemary sprigs for 5 minutes. Use more syrup for stronger herbal flavor. London Dry gin brings a classic crispness.