In a small saucepan, combine dashi, mirin, soy sauce, and sugar (if using). Bring the mixture to a gentle boil, then immediately reduce heat and simmer for 2-3 minutes to allow the alcohol from the mirin to evaporate and flavors to meld.
Remove the tsuyu from heat and let it cool completely. For quicker cooling, place the saucepan in an ice bath. Once cool, transfer the tsuyu to serving cups.
Bring a large pot of water to a rolling boil. Add soba noodles and cook according to package instructions, usually 4-6 minutes, stirring occasionally to prevent sticking.
Immediately drain the cooked soba noodles in a colander. Rinse them thoroughly under cold running water, rubbing gently to remove any excess starch. Continue rinsing until the noodles are completely cold. For extra chill, immerse them in an ice bath.
While noodles are chilling, finely chop green onions and shred nori seaweed.
Arrange the chilled and well-drained soba noodles neatly on a zaru (bamboo tray) or a serving plate. Garnish with shredded nori. Serve with individual cups of chilled tsuyu, chopped green onions, and wasabi on the side.
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