How to Make It
In a soup pot, combine the dark scallion greens with the lemongrass, 2 tablespoons of the ginger and 2 tablespoons of the fish sauce. Add 8 cups of water and bring to a boil. Simmer for 20 minutes. Add the chicken and simmer over low heat just until cooked, about 10 minutes. Remove the chicken. Strain the liquid into a heatproof bowl and let cool. Shred the chicken, add it to the liquid and refrigerate overnight.
In a small bowl, combine the chiles, garlic, sugar, mirin and vinegar with the remaining 2 tablespoons of ginger and 6 tablespoons of fish sauce. Refrigerate overnight.
Bring a large pot of water to a boil. Cook the noodles until al dente, about 6 minutes. Drain and cool under running water. Cut into 6-inch lengths and pat dry, shaking the colander occasionally. Transfer the noodles to a bowl along with the jicama, bean sprouts, sliced scallions and the dressing. Drain the chicken and add it to the salad. Serve with peanuts, cilantro and lime wedges.