Make the soup: bring four cups water to the boil and add the chicken thighs and ginger. Cover and simmer over medium heat for 45 minutes. Remove the chicken and set aside. Add the mushrooms to the water and cook 15 minutes. Season the soup with salt and pepper to taste; it is served on the side.
Remove the meat from the chicken and shred. Season the chicken meat with fish sauce and soy sauce and set aside.
Cook the noodles until just done. While cooking the noodles, heat the oil in a wok or frypan and fry the red onion. Turn off the heat as soon as the onions start to brown; remove and discard the onions.
Drain the noodles and pour the onion-flavoured oil over. Toss well with chopsticks, add the reserved chicken and season with vinegar and sugar. Sprinkle green onions on top and serve with the soup.