1. Cut the sweet potatoes into large bite-sized chunks and cook in simmering salted water for 15 minutes until tender, then drain.
2. Scrub the mussels, pull out the beards and discard any that do not open when sharply tapped.
3. Heat the oil, 250ml water, the shallots, garlic and chilli in a heavy-based pan and bring to the boil. Throw in the mussels, cover tightly and cook for a minute or two, then give the pan a good shake.
4. Lift out any opened mussels with tongs and set aside.
5. Cover again and repeat, discarding any mussels that do not open.
6. Add the sweet potatoes, fish sauce, chilli sauce and sugar to the broth and heat through, stirring.
7. Add the mussels, herbs and lime juice, tossing well.
Serve at once, with lime wedges and steamed rice.