This dish is unbelievably easy. Basically, the idea is to make stock using dashi kombu, and then throw some tofu in. Yes, that's it! Of course you are welcome to add any vegetables to add extra nutrition and color.
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Note: Here are some brief notes on some of the more obscure ingredients. Dashi kombu is dried kelp and is often used to make stock in Japanese cuisine; katsuoboshi have a mild fish flavor and are used in much of Japanese cooking; shichimi togarashi is a Japanese seven-spice chili powder often served with soups; and yuzu kosho is a Japanese hot citrus paste. All of these can be found in Japanese or Asian grocery stores as well as online.
*Note: If not using mitsuba, make sure to cook the leafy greens thoroughly first before using in the stock.
Gently clean the dashi kombu with a damp cloth (do not wash) but leave the white powdery substances that contribute to the umami flavor in the stock.
Place the dashi kombu in a pot and fill with water until 70 percent full. Let the kombu soak for 1-2 hours. (If you are in a hurry, place on the stove and cook over the lowest heat for 15 minutes.)
Meanwhile, place the soy sauce, mirin, and 1 tablespoon of the sake in a small saucepan. Bring to a boil, then add the bonito flakes (katsuobushi). Mix well with chopsticks and cook for 1-2 minutes.
Pour the sauce into a small serving dish. (You don't have to strain; it's OK if some bonito flakes go into the sauce. These leftover bonito flakes have nice flavor.)
When the kombu dashi stock is done soaking, bring to a boil over medium-high heat. Once boiling, add the remaining sake and the salt. Once it returns to a boil, cut the tofu to any size you like and add it to the stock. When the tofu is heated through, add the mitsuba. Serve with the sauce and toppings of your choice while the tofu is hot.