Peel the ginger, then cut into matchstick-like strips.
Remove the outside layer of the lemon grass, then slice it into thin rings.
Thinly slice the spring onion diagonally (the green part too).
Pour the stock into a saucepan, add the ginger, spring onions and lemon grass slices. Bring to the boil, and simmer for 10 minutes.
Meanwhile, slice the mushrooms and cut the chicken into strips about 5 x 1 cm thick.
Add the mushroom and the chicken to the broth, together with the sugar and fish sauce, then cook about 5 minutes.
When the chicken is no longer pinkish, add the spinach and lime juice.
Take the soup off the heat. Taste and adjust the seasoning if needed.