Rub a little butter over the inside of a baking dish, then place the fish fillets in the dish and season with salt. Dot over about half the butter, then pour over a couple of tablespoons of the cider. Cover loosely with a sheet of nonstick baking paper, then place in a preheated oven, 180°C (350°F), Gas Mark 4, for 12–15 minutes until just cooked through.
Meanwhile, heat the remaining butter in a large saucepan. Add the shallot and cook for 3–5 minutes until softened, then add the mussels and remaining cider. Cover the pan and cook for 5 minutes, or until the mussels have opened. Discard any that remain closed. Stir in the crème fraîche and parsley.
Place the fish on serving plates and spoon over the mussels and sauce to serve.