If not already sliced, cut the tuna into sashimi slices about 1/2 inch (1 cm) thick. Thinly slice the green onion.

In a bowl, lightly mix the tempura batter mix (or all-purpose flour) with cold water. Do not overmix; it's best if the batter is still a little lumpy.

Pour plenty of oil into a frying pan or pot and heat until hot (test by inserting chopsticks—if bubbles form, it's ready).

Dip each slice of tuna into the batter, then fry in the oil. Fry until the coating is crispy, then drain excess oil on paper towels.

In a small bowl, mix together the soy sauce, mirin, sake, wasabi, and, if desired, a pinch of chili pepper, shichimi togarashi, or sansho pepper.