Scrub the mussels in plenty of cold water. Scrape off any barnacles and beards with a knife and discard any mussels that fail to open when lightly tapped on a work surface. Rinse well, then drain.
Heat the oil in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the shallots and fennel and cook for 2–3 minutes until they begin to soften. Stir in the harissa, pour in the measurement water and lemon juice and bring to the boil.
Add the mussels and prawns to the tagine, reduce the heat, cover and cook gently for 5–6 minutes, or until the mussels open. Discard any that remain shut. Season, stir in the coriander and serve with couscous, if liked.