Rice Noodle Salad with Chicken

Name: Rice Noodle Salad with Chicken
Ingredients: 4 scallions, white and light green parts thinly sliced, dark greens reserved; 3 lemongrass stalks, smashed with the flat side of a knife; 1/4 cup minced fresh ginger; 1/2 cup Asian fish sauce; 1 1/2 pounds boneless, skinless chicken breasts, halved lengthwise; 2 serrano chiles, seeded and thinly sliced; 1 large garlic clove, minced; 1 tablespoon sugar; 1/4 cup mirin; 1/4 cup rice vinegar; 1 pound pad thai rice noodles; 1 small jicama (about 1 pound), peeled and cut into fine matchsticks; 1/4 pound mung bean sprouts; Peanuts, cilantro and lime wedges, for serving