Slice the bonito slab in 6 mm thick slices. They are sliced thinner than seared bonito so that they'll be well-coated with sesame seeds.
Lightly pan-roast the black sesame seeds. They'll be aromatic when crushed.
Grind the sesame seeds with sesame oil until moist.
Add the soy sauce and sugar and grind again. Make sure it is evenly mixed.
Add the bonito to the sesame seed paste. Adjust the amount of soy sauce and sugar to taste. If the paste doesn't coat well, or if it's too salty, add a little sake.
Wrap in plastic wrap and chill in the refrigerator for at least half an hour, sprinkle with your choice of topping, and serve.
To end your meal, have it as an ochazuke. Arrange the bonito on a bed of rice, sprinkle with your favorite garnishing, pour piping hot tea, dashi soup stock, or hot water.
Story Behind this Recipe
My father, who was born and raised in Kumamoto prefecture, ate bonito this way, so he had my mother prepare this dish. However, upon inquiry, no one in his family ever had this dish, so its origin remains a mystery. There's a high possibility that it's a figment of my father's delusion, so in that sense, its a secret family recipe.
As I pointed out in the recipe: 1. Slice the bonito thinly. 2. Roast the sesame seeds to bring out the aroma. 3. Grind the sesame thoroughly. 4. Adjust the tate with sake. 5. Chill after seasoning, and have it as ochazuke. It's easy to make, even the first time.