Halibut with mussels, potatoes and lettuce

To begin, cut your halibut fillets into 6 equal portions, ensuring you save 200g of trimmings from the skirt and tail (you can ask your fishmonger to do this if preferred). Combine the water, sugar and salt in a pan and heat until the sugar and salt has dissolved. Leave to cool completely, then submerge the halibut portions in the brine and leave to brine for 6 minutes. Drain, rinse, pat dry and then store in the fridge until later

For the sauce, blend or whip the butter and dulse together until fully combined, then set aside in the fridge. Heat the smoked oil in a heavy-bottomed saucepan, then cook the shallots for 5 minutes until soft. Add the mussels and white wine, then turn the heat up to high and cover. Allow to steam for 2-3 minutes until the mussels open, then strain through a fine sieve (or even better, a coffee filter). Reserve the liquid and pick the meat from the mussels. Finely dice the mussel meat and set aside for the mousse

Return the mussel cooking liquor to a separate pan and reduce one-third. Set aside

To make the mousse, add the reserved halibut trimmings and salt to a chilled blender jug and blitz until smooth. Blitz in the egg white, then finally blend in the double cream. Pass through a fine sieve into a bowl and fold through enough of the reserved diced mussel meat to create a pipeable mousse. Transfer to a piping bag

Place each halibut portion on a small piece of baking paper, then pipe the mousse on top of it, around 5mm thick. Refrigerate until needed

For the garnish, fill a saucepan with the olive oil over a medium heat. Add the bay leaves, then the potatoes. The oil wants to be almost boiling, with the potatoes being gently cooked for around 20 minutes. Once cooked, lift the potatoes out of the oil (reserving the oil) and leave to cool slightly before peeling

While the potatoes cool, set up a steamer (or steam oven) and cook the halibut fillets until their internal temperature reaches 50°C. Depending on how hot your steamer is, this should take 5-8 minutes. Leave to rest for a few minutes

To finish the sauce, reheat the mussel liquor and whisk in the dulse butter and mustard over a medium heat until it emulsifies. Pour a little of the sauce over the egg yolks to temper, then whisk this back into the pan over a low heat until thickened and smooth (do not allow to boil, otherwise the eggs will scramble)

To serve, reheat the potatoes in the warm olive oil for 30 seconds, then drain. Divide the lettuce into leaves and add to the warm olive oil leftover from cooking the potatoes. Drain, then dress the leaves and potatoes with a pinch of salt. Add a squeeze of lemon to the lettuce, cut each halibut portion in half, then place 2 halves on each plate. Garnish each dish with the lettuce leaves, 3 potatoes and 3 tablespoons of the sauce