Place the herrings in a bowl, pour over the milk and leave in a cool place for an hour.
Melt 25g (1oz) of the butter and fry the onions until golden.
Drain the herrings and cut into bite sized pieces, add the fried onions and mix with 75g (3oz) of the cream.
Mash the potatoes, adding the remaining butter, cream and egg yolks, season to taste.
Whisk the egg whites to stiff peaks.
Gently fold the eggs whites into the mashed potatoes.
Spread half the potato mixture in a buttered ovenproof dish.
Add the fish mixture, top with the remaining potato mixture.
Bake for 40 minutes.
Garnish with parsley.