Scrub the mussels well with a brush, discarding those that remain open even after they are tapped. Pull off any stringy beards.

Put the oil in a large pan and set over a medium—high heat. When hot, put in the mustard seeds. As soon as they start to pop, a matter of seconds, put in the onion and reduce the heat to medium. Stir and cook for 4-5 minutes, or until the onions have softened. Add the ginger and garlic. Stir for 1 minute. Now add the coconut milk, 250 ml/8 fl oz water and all the remaining ingredients. Stir and bring to a simmer. Cook gently, uncovered, stirring now and then, for 5 minutes. (This much can be done ahead of time.)

Add all the mussels and bring to the boil over a medium-high heat. Cover and cook rapidly for 5 minutes, or until all the mussels have opened.