Wash the fish fillets and dab them dry on kitchen paper. Peel and finely chop the shallots (or onion). Place the cream and mustard in a bowl and mix together.
Lightly butter a casserole dish then lay two of the fish fillets on the bottom. Sprinkle these with a little salt and pepper then add a layer of chopped shallots and a final layer of the cream mix. Lay the next two fillets on top and repeat the process. Finally, sprinkle the top with grated cheese.
Cook in a preheated moderate oven (Mk 6 - 400ºF - 200ºC) for 40 minutes. Serve hot with new potatoes and green beans.