Put 1 liter of water and 2/3 teaspoon salt in a bowl to make a weak saline water. Soak the kazunoko, refresh the saline water every 2 hours, 2 or 3 times (soaking for a total of 6 to 8 hours).
Put all of the ＊-marked ingredients in a pot, then bring to a boil. Add the bonito flakes, then turn off the heat (this is called oi-gatsuo, or extra bonito). When it cools, strain, and put it in a sealable plastic container.
When the kazunoko is sufficiently de-salted, carefully remove the thin, white membranes. Wipe the kazunoko with a paper towel to remove excess water, soak in the marinade, store in refrigerator for a day or more, then it's done.
Garnish with sprigs of sansho or a generous sprinkle of bonito flakes.
Story Behind this Recipe
I called my mom and she told me how to make it. She said you can be approximate for the seasonings, so I measured mine for this recipe.
Be careful when removing the membranes from the kazunoko, since they break apart easily. For de-salting the kazunoko, use water with a little salt instead of plain water; the saline water penetrate the surface, evenly de-salting the kazunoko. De-salt the water at room temperature. It's best to leave a little saltiness. If too much salt is removed, it will become bitter. In that case, soak in strong saline water for 1 to 2 hours to remove the bitterness.