Add the tequila or mezcal (optional), citrus juice and simple syrup to a rocks glass with ice and stir.
Top with the club soda or, for more sweetness, the tonic water, and garnish with a sprig of rosemary.
*Citrus juice: Cut 1 lemon and 1 lime in half, and dip the open sides in turbinado sugar to coat. Grill each half over high heat, with the cut edge toward the flame, until the sugar has charred on the citrus. Once cooled, juice the fruit.