Add the tequila, syrup and juice in a shaker with ice and shake.
Shake and strain into a Collins glass.
Fill the glass to the top with ice, and top with grapefruit soda.
Garnish with a rosemary sprig.
*Rosemary simple syrup: Combine 2 tbsp fresh rosemary leaves (from about 1 sprig) and ½ cup water in a pot, and bring to a boil.
Add 1 cup sugar and stir constantly until fully dissolved. Strain rosemary leaves and let syrup cool to room temperature before using. Do not refrigerate to quicken cooling time, as sugar may crystallize from cooling too quickly.
Once cool, use immediately or store refrigerated in an airtight container for up to 2 weeks.