A classic bright and lemony French 75 cocktail with juniper berry- and rosemary-infused simple syrup. When making the juniper simple syrup, use culinary juniper berries, available at Whole Foods and other specialty markets, and not juniper from the landscape.
To make syrup, heat water and sugar in a small saucepan over medium heat. When sugar has dissolved, add juniper berries and rosemary. Stir until things get steamy. Off the heat and allow the mixture to steep for 15 minutes or so.
Combine gin, lemon juice, and simple syrup in an ice-filled shaker, and shake for a minute or so. Strain into Champagne flutes. Top off with Prosecco or Champagne, and finish with a thin strip of lemon zest.