Combine lemon juice, 1 cup water, and brown sugar in a saucepan over medium heat. Stir until sugar dissolves, 2 or 3 minutes. Remove from heat and let cool. When cooled to room temperature, pour into a pitcher.
Pour tonic water, cold brew concentrate, and 1 1/2 cups water into pitcher; stir. Refrigerate until chilled, about 1 hour.
To add smoked rosemary flavor, place rosemary in a saucepan. Use a kitchen torch to singe rosemary. Immediate invert a pitcher over the smoke. Let sit for 2 minutes, or until the smoke settles. Add chilled coffee-lemonade mixture and cover about 5 minutes.
This recipe was developed by this Allrecipes Allstar as part of a campaign sponsored by Gevalia.