Clean the mussels. Heat a little water in a large pan, add the mussels and put the lid on. Steam for a few minutes until the mussells open. Discard any mussels that don't open. Put the mussels and their juices to one side.
Heat a drizzle of olive oil in a frying pan and gently soften the onions, garlic, ginger and chillies.
Add the vinegar and the sugar, stir well to dissolve, and taste to make sure you have got that sweet and sour balance. Add a little more vinegar or sugar if necessary. Season with a few drops of fish sauce and simmer for 2-3 minutes.
Tip the mussels and their juices into the frying pan and stir well to create a sticky coating over the mussels.
Squeeze in the lime juice, add the herbs and serve.