Whisk together mustard and soy sauce in a small bowl. Bring vinegar, red wine, and port to a boil in a saucepan over medium-high heat. Cook 10 minutes or until liquid is reduced to 1/2 cup. Reduce heat to medium-low; add mustard mixture and cream. Simmer 3 minutes, stirring occasionally. Reduce heat to low; whisk in butter, 1 piece at a time. Pour through a fine wire-mesh strainer into a bowl; keep warm.
Process nori furikake in a food processor until finely ground. Transfer to a shallow dish. Cut fish into 6 (2" x 2" x 8") loins. Spread a thin layer of miso all over loins; dredge in ground nori furikake.
Heat 1 tablespoon oil in a large skillet over medium-high heat until hot. Add 2 tuna loins, and cook 1 to 2 minutes on each side or to desired degree of doneness. Repeat with remaining oil and tuna. Slice fish; serve with sauce.
Nori furikake is a Japanese seasoning mix that includes seaweed and sesame seeds, and commonly jazzes up rice, soups, pasta, and eggs.