Season the flour with salt and pepper. Beat the egg with the milk and the mustard. Rinse and dry the herring fillets. Dip into the beaten egg mixture and then in the seasoned flour.
Melt the butter in a large frying pan and fry the fillets over a moderate heat for 3—4 minutes on either side and until a light golden brown.
Meanwhile, to make the sauce, whisk together all the ingr