With a sharp knife, cut each of the fillets into three pieces. Make a shallow “x” in the skin of the mackerel over the thickest part of each piece of fish.
Combine the soy sauce, rice wine, and syrupy rice wine in a deep glass or ceramic dish, and marinate the fish, turning it several times, for at least 3 hours, and up to 12 hours if refrigerated.
Remove the fish from the marinade (reserve the marinade for later), and place it skin side down on a flat rack, the kind you put cakes or cookies on to cool. I set my rack over a tray to catch any marinade drippings and place it in a cool spot out of direct sunlight. In my city apartment, that means the dining room table. Sprinkle the flesh side of the fillets with the black sesame seeds. Allow the fish to air-dry for at least 3 hours and up to 7 or 8 hours, until the surface of the fish is dry to the touch and no longer sticky.
Place the fish, skin side up, on a rack or disposable foil broiling pan. Preheat your broiler to the highest heat possible, and broil the fish 2–3 inches from the source of heat for 2 minutes. (The fish will begin to curl slightly and the flesh will lighten in color.)
Turn the fish over and continue to broil for 5-6 minutes. The fish will be a beautiful burnished color, with the surface ever so slightly charred in some places.
Serve two pieces for each serving, with lemon wedges on the side. Serve the fish hot, or let it cool to room temperature.