Tear lime leaves in several places from edges toward the centers. Cut lemongrass into 1 inch pieces; bruise with the flat side of a knife. Stir lime leaves, lemongrass, chicken, coconut milk, mushrooms, sweet pepper, chopped onion, nam pla, and snipped cilantro into broth mixture. Bring to a boiling; reduce heat. simmer, uncovered , for 5 minutes.