Pound or finely chop the garlic, chilli, ginger and galangal.
Heat the oil in a large pan over medium heat.
Add the onion slices, the lemongrass & the pounded ingredients, stirring and cooking for a minute, you'll get a lovely aroma at this stage.
Pour the coconut milk in, add the soy sauce and lime leaves too. Stir it all in.
Add the mussels in, give them a very quick stir, cover & leave to steam on the same heat for about 3-4 minutes, giving it a shake or two during this time. At the end of the 3-4 minutes, they will be done.
Tip everything onto a warm serving platter, squeeze the lime juice all over, scatter with the fresh coriander and serve immediately.