Look for Kaffir lime leaves and galangal—also known as Thai ginger—at ethnic markets and gourmet grocers. If you can't find them, use an extra 1/4 cup chopped peeled fresh lemongrass to flavor the broth. You can use regular ginger in place of galangal.
2 cups fat-free, lower-sodium chicken broth
3/4 cup chopped peeled fresh lemongrass
6 Kaffir lime leaves, torn
5 (1/4-inch) slices fresh galangal
1 1/2 tablespoons red curry paste
1 (4-ounce) package presliced exotic mushroom blend (such as shiitake, cremini, and oyster)
8 ounces skinless, boneless chicken breast, cut into bite-sized pieces
1 (13.5-ounce) can light coconut milk
1 tablespoon Thai fish sauce (such as Three Crabs)
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.
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.