Marinate the Fish: Combine the marinade ingredients together in a large zip-top bag. Add the fish. Squeeze as much air out as possible; then refrigerate for 2 - 4 hours.
Prepare the Curry Mustard Sauce: Mix the mustard sauce ingredients together in a small bowl; add salt and pepper to taste. Set aside.
Prepare the grill, gas, charcoal, grill pan, to high. Remove the fish from the marinade. Season with salt and freshly ground pepper. Rub the grates with the cut side of the potato, then add the tuna, grill, about 3 minutes per side, leaving the center pink. DO NOT OVERCOOK.
Slice the fish and serve with some of the mustard sauce. Garnish with cilantro leaves, if desired.
You may not need all of the mustard sauce, but it's great on sandwiches too