In a large dish, mix all of the marinade ingredients together. Add the tuna steaks to the dish, and turn them until they’re well-coated. Set aside, and let them rest at room temperature for about 20-30 minutes.
Place the cooked steaks on a cutting board, and slice against the grain into ½-inch thick slices. Transfer the steak slices to a serving plate and drizzle with the reserved maple mustard marinade. Serve immediately.