Cumberland Stuffed Herrings with Mustard Sauce

Yield: 4 servings. Preheat oven to 180 C / 350 F / Gas 4 Put the roes in a small saucepan with the milk. Bring to the boil and simmer gently for 5 minutes. Drain, reserving the milk. Finely chop the roes. Open out the fish on a board, inner side down and press lightly down the middle to loosen the backbone. Gently ease the backbone away. Mix together the breadcrumbs and onion and add the chopped roes. Spread on the open herrings. Fold the herrings over to enclose the stuffing. Cover and bake for 20 minutes until tender. Meanwhile, put the butter, flour and the reserved milk in a saucepan. Heat, whisking continuously, until the sauce thickens, boils and is smooth. Simmer for 1-2 minutes. stir in the mustard and vinegar and serve with the herrings.