Fill a cocktail shaker with ice. Add gin, lemon juice, and rosemary simple syrup. Shake well for about 15 seconds.
Strain into a Collins glass with fresh ice.
Top with sparkling water and stir gently.
Garnish with an orange slice and, if desired, a rosemary sprig. Serve immediately.
Notes
To make rosemary simple syrup: simmer equal parts sugar and water with rosemary sprigs, cool, and strain. Adjust syrup for sweetness and rosemary to taste. Chilled sparkling water keeps it crisp.