Wash the salt herring. If they are whole, fillet and discard the backbone. Cut the meat into chunks and soak overnight in cool water, changing a time or two. Drain and rinse the pieces. Layer the fish in a deep bowl, alternating with a layer of onions. Cover with water, then pour the water into a measuring bowl and note the amount. Discard the water and place an equal amount of vinegar into a saucepan, then stir in the brown sugar and black pepper. Bring the vinegar to a light boil, then pour it over the fish. Cover the container, then cool it in the refrigerator for several hours. Serve cold.