Preheat the oven to 180°C/350°F/Gas 4. Lightly butter the ovenproof dish. Season the fish with salt and freshly ground pepper. Arrange the fillets in a single layer. Mix the grated cheese with the mustard and cream and spread carefully over the fish. Cook in the preheated oven for about 20 minutes or until the fish is cooked and the top is golden brown. Flash under the grill if necessary. Serve with new potatoes and a good green salad.