Thinly slice as much of lemon grass stalks as possible, discarding dried thin upper portion. In a saucepan simmer water with lemon grass, covered, 5 minutes. Add mint and simmer, uncovered, 1 minute. Remove pan from heat and add sugar, stirring until dissolved. In a blender puree mixture and strain through a fine sieve into a bowl, pressing hard on solids. Chill syrup, covered, until cold and freeze in an ice-cream maker. Sorbet may be made 1 week ahead.