Using a sharp knife, cut the tuna loin into thin, even slices, about 1/4 inch (0.6 cm) thick. Arrange the slices on a serving platter.
In a small bowl, combine the soy sauce, sesame oil, rice vinegar, and freshly grated ginger. Mix well.
Drizzle the sauce over the sliced tuna or serve it on the side for dipping. Sprinkle toasted sesame seeds and thinly sliced green onions over the tuna if desired.
Serve the sashimi with wasabi and pickled ginger on the side. Optionally, serve with additional soy sauce for dipping.
Enjoy your meal!