Stud a couple of the onion slices with the cloves. Put the onion slices in a large pan with the bay leaf, 15 fl oz milk, the cream, and all the fish. Bring just to the boil and simmer over a low heat for 5 minutes.
Lift the fish out on to a plate and strain the cooking liquid into a jug. When the fish is cool break into large flakes, discarding the skin and bones. Spread over the base of a shallow 3 pint ovenproof dish. Cover with the sliced eggs.
Melt 2 oz of the butter in a pan, then add the flour and cook for one minute. Remove from the heat and gradually stir in the reserved cooking liquid. Return to the heat and bring slowly to the boil, stirring.
Simmer gently for 10 minutes then remove from the heat. Stir in the parsley and season with nutmeg, salt and pepper. Pour over the fish and leave to cool, then chill in the fridge for an hour.
Boil the potatoes for 15-20 minutes. Drain, mash and add the remaining butter and the egg yolk. Beat in enough of the remaining milk to form a soft spreadable mash and season.
Preheat the oven to 200 C, Gas 6. Spoon the mash over the filling and ruffle up the surface with a fork. Bake for 35-40 minutes until golden then serve immediately.