Fillet the ready-to-cook herrings, cut the fillets in half, season lightly with salt and brush both sides with mustard and roll in the parsley.
Prepare a batter with the other ingredients, folding in the egg whites until stiff.
Pull the fillets through the batter and bake until crispy.
Serve with potato salad or brown bread and leaf salad, e.g. a bell pepper salad, as well as pearl onions and/or tomato salad.