Make rosemary simple syrup (do this ahead if you can): Combine water and sugar in a small saucepan over medium heat. Stir until the sugar’s dissolved. Toss in the rosemary sprigs, take it off the heat, and let it steep for about 30 minutes. Strain, cool it down, and stash any extra in the fridge for up to two weeks.
Prep your glass—just fill it up with ice cubes.
In your shaker, drop in a rosemary sprig and give it a gentle muddle to release those oils.
Add gin, lemon juice, and rosemary syrup to the shaker.
Fill with ice and shake it like you mean it for 15-20 seconds, until it’s nice and cold.
Strain into your ice-filled glass.
Top with club soda and give it a gentle stir.
Garnish with a rosemary sprig and a lemon wheel. That’s it.