Grease 4 scallop shells or individual ovenproof dishes.

Put the frozen fish steaks into a heavy-based frying-pan. Pour over the water, then add the onion and bay leaf and season to taste with salt and pepper. Bring to the boil, then lower the heat, cover and simmer for 15 minutes.

With a fish slice, lift the fish and onion from the liquid. Strain the liquid and reserve.

Leave the fish until cool enough to handle, then flake the flesh. Divide the fish between the prepared dishes and set aside.

Heat the grill to moderate.

Melt 25 g (1 oz) of the margarine in a small saucepan, sprinkle in the flour and stir over low heat for 1 - 2 minutes until straw-coloured. Then remove from the heat and gradually stir in the reserved fish liquid and then the milk. Return to the heat and simmer, stirring, until thick and smooth. Remove from the heat and stir in half the grated cheese. Stir vigorously until the cheese has melted, then season to taste.

Spoon the cheese sauce over the fish in each shell, dividing it equally between them. Mix the breadcrumbs with the remaining cheese and sprinkle over the sauce in each shell. Dot with rest of margarine.

Grill until golden brown and heated through and serve at once.