Add tequila, Campari, rosemary simple syrup (see easy instructions below), and lime juice to an ice-filled cocktail shaker.
Shake well then pour into an ice-filled glass (we used an ice sphere with rosemary frozen into it).
Rosemary simple syrup: Bring all ingredients to the boil in a small saucepan while stirring, then immediately lower heat, and cook until all sugar is dissolved. Leave to cool and infuse (up to 12 hours). Remove rosemary. Syrup can keep for up to 2 weeks.