In a small, heavy pot, stir the water into the stevia until they are combined and the mixture looks foamy. Mix in the rosemary. Cook over medium-high heat until the mixture boils, stirring 3-4 times, 4 minutes. Cover and immediately set aside to steep for 20 minutes. Strain the syrup through a fine strainer into a small bowl; makes 1/2 cup. Cool, then chill the syrup. Tightly covered in the refrigerator, Rosemary Stevia Syrup keeps for 2 days.
Fill a 12–ounce glass halfway with ice cubes. Add the orange juice, lemon juice, and Stevia Rosemary Syrup. Top off with club soda. Garnish with the fresh rosemary, if desired. Serve immediately.