1: First, cook the mussels. Wash the mussels in running cold water. Discard any mussels that float, any with cracked shells and any open ones that do not snap shut when tapped. Heat a large saucepan over a high heat. Add the mussels and a splash or water, cover the pan and cook for 5–6 minutes, shaking the pan occasionally, until the shells have opened. Drain the mussels and discard any that have not opened. When the mussels are cool enough to handle, remove them from the shells and set aside.
2: Heat the rapeseed oil in a large frying pan over a low heat. Season the fish and dust the skin sides with flour, shaking off the excess. Then place the fillets into the pan, skin side down, and fry for about 8 minutes until the skin becomes crispy.
3: Meanwhile, heat another frying pan over a medium heat. Add the bacon and fry, stirring occasionally, for 4–5 minutes until it becomes crisp and renders out its fat and flavour.
4: Add 20g of the butter to the bacon and heat until it melts and becomes a lovely hazelnut-brown colour. Take the pan off the heat and stir in the sea vegetables with a small splash of water. Just let them gently wilt. Add the mussels to warm through.
5: Flip the fillets over when the skins are crispy, then throw in the remaining 20g butter. Squeeze over the juice of the lemon and baste the sea bass with the buttery pan juices.
6: Remove the sea vegetables and bacon from the pan and pat with kitchen paper. Divide them between 2 plates, place the fish on top and spoon a little of the cooking butter over the top. Serve immediately.