Combine all brine ingredients in a 1 1/2 quart saucepan and bring to a boil. Stir occasionally until sugar is completely dissolved. Let cool, cover and reserve.
Completely submerge and soak 12 preserved herring fillets in cold water overnight to extract salt. Remove fillets and pat dry. Arrange in a shallow pan and cover completely with pickling brine. Refrigerate overnight. The next day, remove herring from brine and dice into 3/4-inch pieces. Serve with the Mustard sauce.
Combine the sweet and Dijon mustard, sugar, vinegar, coffee, salt and pepper in a food processor. Drizzle canola oil into the mixture in a very slow, steady stream, until a thick consistency is formed. A whisk may also be used. Refrigerate overnight;
Copyright 2016 Television Food Network, G.P. All rights reserved.