Combine the rice vinegar, salt, and sugar and mix thoroughly until dissolved.
Add the freshly steamed rice to a bowl and fold in the rice vinegar mixture.
Line a tray with cling film and press the rice down firmly into an even layer, then place in the freezer for a couple of hours.
For the filling, finely slice the scallion whites and give the tuna a fine dice. In a bowl, combine the tuna, scallions, light soy sauce, sesame oil, and sugar, then set aside.
Remove the rice from the tray and cut it into rectangles, setting them aside.
Add oil to a pan and shallow fry the rectangles until golden brown on both sides. Remove and rest for at least 10 minutes.
Place the tuna mixture on top of the crispy rice, cut into little squares, and arrange on a plate.
Optionally garnish with soy glaze, sriracha mayo, sesame seeds, and scallions. But it's your food, you eat it how you like.