Remove and discard any dried outer leaves from lemongrass. Chop lemongrass stalks, and transfer to a large saucepan. Add the water and sugar; bring just to boiling. Remove from heat and let stand for 15 minutes. Add tea; let stand 5 minutes more.
Pour lemongrass-and-tea mixture through a fine mesh sieve or cheesecloth into a glass bowl; discard solids. Let liquid cool. Stir in lemon juice, if desired. Cover and refrigerate for 2 hours or until chilled. Transfer mixture to a 13x9x2-inch baking pan. Cover and freeze for 8 hours or overnight until firm.
Break frozen mixture into large chunks. Place about one-fourth of the mixture in a food processor or blender container. Cover; process or blend until smooth but not melted. Transfer to a medium bowl; repeat with remaining mixture. Cover and freeze.
To serve, scoop several small scoops of sorbet into chilled tea bowls. Garnish with mint leaves. Serve immediately. Makes about eight 1/2-cup servings.