Wash and
clean the mackerel, slash the sides of each 2-3 times. Peel and cut the
onions into very thin slices and put them into a well-buttered ovenproof
dish. Lay the fish on top and dot with the butter. Season with salt and
pepper and bake in a moderate oven (190C, 375F, Gas 5) for 35 minutes.
Pour the cream over. Add more seasoning if necessary and grate a little
nutmeg on top. Return to the oven for a few minutes to heat the cream.
Do not let it boil.