In small saucepan, stir together water, agave, rosemary, and juniper berries over medium-high heat until simmering. Reduce heat to low and let mixture cook, uncovered, for 20 minutes, stirring occasionally. Strain simple syrup into heatproof container and refrigerate until chilled.

To lowball glass, add clementine juice and 1 Tbsp rosemary-juniper simple syrup. Stir to combine. Add ice, if desired, before adding sparkling water and orange bitters. Gently stir to combine. Garnish with a sprig of rosemary and a slice of clementine. Serve.