Put the mussels straight from the freezer into a large saucepan with the wine. Bring to the boil and boil for 5 minutes. Then remove the mussels with a slotted spoon and remove the mussels from the shell. Save the shells and 1 cup of the juice you boiled the mussels in.

Cut the mussels into small pieces in a bowl. In a saucepan, melt the butter over medium-high heat. Add the onion and sauté, stirring until it softens and begins to brown.

Add the flour and sauté, stirring for 1 minute. Add the cream, mustard, lemon juice and the reserved stock. Stir until the mixture boils and becomes thick like a béchamel. Remove from the heat. Add the mussels, headcheese and chives. Season with salt and pepper.

For the topping, mix all the ingredients in a bowl.

Take a baking tray and line it with crumpled silver paper. Place the shells on the baking tray. The use of the crumpled silver paper is so that the shells sit straight without tipping and the filling comes out with them. So make sure they are as straight as possible.

Fill each shell with the filling. Sprinkle with the topping. At this stage you can cover them with plastic wrap and store them in the refrigerator for 12 hours.

To bake them, preheat the oven to 175°C. Place the baking tray in the oven for 20-25 minutes until golden brown.

Serve immediately on a bed of salt.