Boil or steam the potatoes, peel and cut into slices, not too thin. Skin, bone and fillet the herrings, which should have been soaked in milk for 2 hours, and chop them. Put the potatoes and the chopped herrings on a buttered fireproof dish and pour the following sauce over them: Chop the onion and cook in the butter till slightly yellow, but without browning. Add a cup of warm milk, the stock, and the roe, previously blanched and rubbed through a sieve. Season with salt and pepper and pour over the potatoes. Sprinkle with grated Parmesan cheese and breadcrumbs, dot with pats of butter and put in a moderate oven for 20 minutes.