Peel and quarter the potatoes, then cook them in salted water until they start to crumble. Pour off the water, heat the milk and add to the potatoes and use the masher to form a light pulp. Season with nutmeg and pepper.
Clean and remove the herrings, debone. Do not throw away the roe and milk, but wash them as well. Then season herrings, roe and milk and roll in flour. Fry in a pan with simmering oil until crispy.
Incidentally, brown the onion rings in a second pan with melted butter until crispy.
Garnish the mashed potatoes with the roasted onion rings, distribute the melted butter on the puree and serve the fish with it.
I can highly recommend raw sauerkraut as a vegetable side dish.