The secret seasoning called Shiro Dashi (白出汁) is a seasoned dashi stock that is made of sake, mirin, light soy sauce, salt, bonito flakes and konbu (kelp). It is very similar to the flavour of the dipping sauce for Tempura and cold noodles such as Zaru Soba and Sōmen. But Shiro Dashi is condensed and you are meant to use it by diluting it with water/boiling water.
Add the remaining ingredients to the saucepan, bring down the heat to minimum so that it simmers extremely gently (almost like keeping the liquid warm for 5 minutes.
Turn the heat off and let it cool for 10 minutes.
Place two layers of muslin or kitchen paper over a sieve and place it over the top of the container.
Put the shiro dashi liquid with bonito/kelp through the sieve and collect clear liquid. Squeeze the muslin/kitchen paper to get the liquid out of the bonito flakes.
1. Store in a container/bottle. It keeps up to 1 month in the fridge.
Add water and bring it to a boil to make a clear soup.
Dilute 2 tsp shiro dashi with 3 tbsp water as the flavour base for Dashimaki Tamago (Japanese Rolled Omelette).
1. The final quantity you make depends on the evaporation rate and the type of bonito flakes you use. My bonito flakes were large pieces (larger than those you buy in a pack at the grocery stores/supermarket) and they absorbed quite a lot of liquid.
2. The original Shiro Dashi uses white soy sauce instead of light soy sauce. You can buy white soy sauce at Japanese grocery stores but I used light soy sauce instead. You could also use normal soy sauce.
The difference is the colour of the shiro dashi, which will then become the colour of the sauce for the dish you are going to make. I personally think that light soy sauce is quite OK since this is a home cooking recipe.
3. You can adjust the ratio of mirin and salt to make it sweeter or saltier to suit to your palate.