1. Wash and cook the potatoes in boiling salted water for 15 minutes, then cut them into pieces of 2 cm.
2. Peel and grate the ginger.
3. Add the ginger, lemongrass, and chicken stock to 1 litre of hot water. Bring to the boil. Cook for 20 minutes over a low heat.
4. Slice the chicken into thin strips. Over a high heat, add the chicken and potatoes and cook for 5 minutes.
5. Wash the carrots and mushrooms. If necessary, peel the mushrooms and slice them into thin strips. Cut the carrots into small sticks.
6. Thinly slice the scallions.
7. Add the carrots, mushrooms, scallions and soy sauce and boil for another minute.
8. Serve immediately.

Tip: Serve the stock hot with a dash of lime juice and a little fresh coriander for added freshness.