Wash the mussels under cold running water discarding any that are broken or open. Clean well and pull off any beards. Drain.
Heat the oil in a large saucepan over a medium heat. When hot, but not smoking, add the garlic and fry for a few seconds until it just begins to colour.
Add the mussels, the harissa and half the coriander. Cook, covered for 30 seconds to a minute until the mussels begin to open and release their juices, shaking the pan as you go.
Then cook uncovered for a further 1-2 minutes until the shells have opened fully. Throw away any that are still closed.
Check seasoning and serve immediately with the remaining coriander, and extra drizzle of olive oil over the top and bread.