In a large pot (with a lid), add water, stock concentrate, lime leaves, lemongrass, ginger, coriander root, chilli, fish sauce and sugar. Bring to the boil then add chicken. Return to the boil then reduce heat to a simmer, cover pot with lid and cook for about 20 minutes, until chicken is cooked through.
Meanwhile, soak rice noodles in hot water until tender, or as instructed on packet. Strain off water and set aside.
Remove chicken from soup, shred and set aside. Season remaining soup with lime juice then strain.
To serve, divide rice noodles evenly between bowls then top with shredded chicken. Sprinkle each bowl with spring onion, carrot, bok choy and sprouts, then pour over the hot soup. Garnish with coriander and serve immediately.