• 225 g onion, chopped
• 8–10 tablespoons olive oil
• 1.35 kg mussels
• 300 ml dry white wine
• 1–⅓ tablespoons shallot or spring onion (scallion) chopped
• 2 sprig of parsley
• 2 sprigs of thyme
• 560 g cooked potatoes
• sea salt and freshly ground pepper
• 55 g buttered crumbs
• 2 large cloves garlic, peeled and crushed
• 2–2½ tablespoons chopped parsley
• 30 g finely grated Parmesan cheese

Sweat the onions in half the olive oil in a heavy-bottomed saucepan until soft and golden brown (about 7–8 minutes).

Meanwhile check that all the mussels are tightly closed, wash them and put them into a stainless steel saucepan with the wine, shallot and herbs, cover and cook on a medium heat. As soon as the shells open remove to a tray to cool. Strain the mussel cooking liquor, take the mussels out of the shells, remove the beards and put the mussels into the liquid.

Slice the potatoes, toss in the remaining olive oil in a frying pan, season with sea salt and freshly ground pepper, then layer the potatoes, mussels and onions in a gratin dish and pour on some of the mussel liquor to come a quarter way up the dish. Mix the buttered crumbs with the garlic, chopped parsley and cheese. Sprinkle over the top.

Bake in a moderate oven, 180°C/350°F/Gas 4, until hot and bubbly – 15–20 minutes. Flash under a grill to crisp the top if necessary. Serve with a good green salad.