Make the sauce.
Combine the soy sauce, mirin and bonito flakes in a pan over low heat. Use plenty of bonito flakes.
It'll start simmering right away. Let it simmer for a bit, then take off the heat and let cool. When it's cooled, strain off the bonito flakes. This is what makes Tosa-style soy sauce.
Beat the egg yolk in a bowl, and add the Tosa soy sauce from Step 2 little by little.
The sauce is done!
Serve the chopped fatty tuna (toro) on a plate.
Finely chop the green onion, and snip the nori seaweed finely with kitchen scissors.
Mix everything together just before eating. Make a mound of rice in a bowl, then top with the minced tuna and green onion, drizzle on some of the sauce and top with the nori seaweed. Add wasabi to the sauce if you prefer.
Story Behind this Recipe
The sauce my mother tops on her negitoro don is delicious, so I asked her to teach it to me. I adjusted the amounts a little bit.
I've listed an easy to make amount for the sauce, which is about 3 servings' worth. If you don't have a small pan, double the amount. Use the leftover sauce for natto, tofu or drizzled in miso soup. You could also use it to season stir-fries and simmered dishes. I recommend using it for an enoki mushroom stir fry. Avoid reducing the sauce.