Throw away opened mussels. If necessary, scrub the outside of the mussels clean with a brush. Soak the mussels in cold water for 10 minutes, then drain.
Heat the oil in a wok or large saucepan. Add onions, garlic, lemon leaves or lemongrass and chillies and sauté everything for 4 minutes over medium heat, stirring occasionally. Then deglaze with wine and fish sauce and cook for another 3 minutes.
Add the mussels, shake well and cook covered for 3 - 4 minutes over a reduced heat until the mussels open. Do not cook too long or they will become hard.
Throw away any mussels that have not opened after 4 minutes. Add the basil, mix everything in well and serve with steamed rice or simply with baguette.