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Bring 600ml water to a boil in a pot with the kelp. Once bubbles start forming, remove the kelp, then stir in 1 tsp hondashi and 1 tsp soy sauce to make the broth.
Grate the radish on a grater (use the milder top half).
Cut the scallion and soak it in water.
Boil the buckwheat noodles in water, adding cold water about three times along the way, then drain and rinse in cold water.
Place the buckwheat noodles from step 4 in a bowl and pour over the broth from step 1.
Top with the radish, tuna, and scallion. Sprinkle with everything bagel seasoning.
Add a little chili oil on top if you like.