Bring 8 cups water to a boil in a medium pot over high heat. Meanwhile, peel 2 or 3 tough outer layers off lemongrass and mash core with a meat mallet or bottom of a small, heavy skillet. Cut into 2-in. pieces.
Remove boiling water from heat, add lemongrass, and let steep, covered, 2 minutes. Crush 6 cups mint in your hands and add to pot. Cover and let steep 5 to 8 minutes. Pour through a strainer into a large heatproof pitcher and stir in honey if you like. Divide among 8 cups and top each with a sprig of mint.
Note: Nutritional analysis is per 1-cup serving.