1. Bring broth to a boil in a medium saucepan over medium-high heat; stir in lemongrass, lime leaves, and galangal. Reduce heat, and simmer 5 minutes. Remove from heat; let stand 30 minutes. Strain through a sieve over a bowl; discard solids. Return broth to pan; add curry paste, stirring with a whisk. Bring to a simmer over medium-high heat. Add mushrooms; cook 2 minutes or until tender. Stir in chicken; cook 3 minutes or until chicken is done. Add coconut milk, stirring well to combine. Stir in fish sauce and sugar, stirring until sugar dissolves. Remove from heat; stir in onions, juice, and 1/4 cup cilantro.
2. Cook noodles according to package directions; drain. Add noodles to coconut milk mixture. Ladle 1 cup soup into each of 6 bowls; sprinkle evenly with remaining 2 tablespoons cilantro.