Put milk into a bowl and soak the herring in it overnight. Remove the and pat dry being sure to remove any and all bones.
Coarsely chop the herring. Fry the onions in 2 tbps of butter until golden.
Pan-fry the ground meats separately and place into a food processor/blender. Add the onion, herring, & potatoes. Chop/blend until a smooth mixture is formed.
Stir in the sour cream & egg yolks then add the spices. Beat the egg whites until they are stiff but not dry. P
reheat the oven to 400F-degrees & grease a 2 qt. baking dish.
Fold in the egg whites at this point and turn the mixture into the greased baking dish.
Sprinkle with the bread crumbs and parmesan cheese, dot with the remaining butter, & then bake for 40 minutes.