In a small bowl, stir together the mayonnaise and wasabi powder until well blended, then stir in green onion; set aside.
Rub the tuna steaks with 1 tablespoon of the sesame oil, then sprinkle with 1/4 teaspoon of the five-spice powder.
Let the wasabi mayonnaise and the tuna steaks stand for 15 minutes at room temperature to allow the flavors to blend, or cover and refrigerate for up to 1hour; if refrigerating, return to room temperature before grilling the tuna and assembling the panini.
Preheat the sandwich grill.
Arrange the tuna steaks in the grill, close the top plate and cook until seared on the outside and about 1/4 inch into the interior, 8-10 minutes (time may be less, so watch closely).
Transfer immediately to a plate; leave the grill on and clean the grill plates.
Place the rolls, cut side down, on a work surface and brush the crust sides with the remaining 1 tablespoon sesame oil, then sprinkle with the remaining 1/4 teaspoon five-spice powder.
Turn and spread the wasabi mayonnaise on the cut side of each roll, then layer 1 tuna steak, half of the pickled ginger, and half of the watercress over the mayonnaise on the bottom half of each roll.
Cover with the top halves of the rolls, mayo side down, and press to pack gently.
Place the panini on the grill, close the top plate, and cook until the bread is golden and toasted, the ginger and tuna are warmed through, and the watercress is just slightly wilted, 2-4 minutes.
The tuna should not cook much further; place on warmed plates and serve at once.