Preheat oven to Gas Mark 4, 180°C (350°F).
Combine the lemon juice, cider, shallots, 1 sprig of thyme, bay leaf and black pepper in a saucepan. Cook for 8-10 minutes until the liquid is syrupy. Add the chicken stock, cream and apples. Heat until it is simmering gently.
Allow to cook until the mixture is reduced by half. This will take about 10 minutes. Remove from the heat. Remove the sprig of thyme and bay leaf and discard. Purée the sauce until smooth. Then strain and return it to the saucepan. Stir in the mustard and chives or parsley and tast for seasoning. Keep warm.
Meanwhile season the fish with a little salt and pepper and sprinkle over the remaining thyme leaves. Place the fish in a lightly oiled roasting thin. Cook in the preheated oven for about 10-12 minutes until it can be flaked easily with a fork.