Place the fish on a board and use a sharp knife to cut down either side of the backbone to form 2 fillets, then chop the fish into bite sized chunks.
Heat 1 tablespoon of the butter in a frying pan and add the fish pieces. Cook over a medium heat for about 3 minutes until the fish is just opaque.
Divide the fish between four ovenproof dishes.
Heat the remaining butter then add the chopped shallots and cook over a medium heat, stirring occasionally for 5 minutes until they are softened.
Pour in the stock, wine and cream then bring to a simmer.
Let the sauce gently bubble and reduce by half.
Stir in the mustard, half of the cheese and parsley.
Check the seasoning then add pepper and salt to taste.
Divide the thermidor sauce between the dishes and sprinkle over the remaining cheese.
Preheat the grill to medium and grill for about 4 minutes until the top is golden and crusty.
Disclaimer:The nutritional information provided is approximate and is calculated using online tools. Information can vary depending on various factors, but we have endeavoured to be as accurate as possible.