Peel the potatoes and cook them in lightly salted water until tender - about 20 minutes.
Meanwhile, prepare the sauce: peel the ginger and finely grate it. Remove the seeds of the chilli, chop the flesh finely. Finely chop the herbs.
ln a small bowl combine the fish sauce, lime juice, oil, garlic and ginger. Season to taste.
When the potatoes are cooked, rinse with cold water, then cut into thin slices (lengthwise).
Place a layer of potatoes in to a shallow serving dish, then pour 2-3 tablespoons of dressing over them.
Repeat these until you used up all potato slices. Don't stir it!
Pour the remaining dressing over the layered potato slices then sprinkle the herbs on top.
If you have time leave the dish to infuse the flavours for a while.
Serve at room temperature.