Both kombu (a savory kelp) and bonito flakes (dried, smoked skipjack tuna) are high in flavorful umami compounds, which help make the broth in this dish (called dashi) rich and smoky. While it may require a trip to a health-foods store or Asain grocery to find these ingredients, the broth itself is super easy to make and freezes nicely.
Line a fine mesh strainer with a double layer of cheesecloth and set over a large bowl.
Bring a large pot of water to a boil. Add the soba and cook according to the package directions. Drain and rinse under cold water. Set aside.
Meanwhile, put the kombu in another large pot with 6 cups of water. Bring to a simmer over high heat; adjust the heat and simmer the kombu for 5 minutes. Remove the kombu, then add the bonito flakes to the pot. Bring back to a simmer and cook for 3 minutes. Strain the mixture through the cheesecloth-lined strainer, return the liquid to the pot and stir in the mirin, soy sauce and vinegar. The broth can be used immediately, or cooled and refrigerated up to 3 days or frozen up to 1 month.
Put the sesame seeds in a small skillet and toast over medium heat until lightly golden, about 3 minutes, swirling the pan frequently so they toast evenly. Set aside. The sesame seeds can be toasted and stored at room temperature in an airtight container for up to 2 days.
Cut the leafy tops off of the bok choy and set aside. Cut the stems into bite-size pieces. Add the bok choy stems and ginger to the broth, bring a simmer and cook until the bok choy is just tender, about 5 minutes. Add the bok choy tops, shrimp and leek to the broth, bring back up to a simmer and cook until the shrimp are pink and just cooked through, about 3 minutes.
Divide the soba among 4 deep bowls. Ladle broth, vegetables and shrimp into the bowls and sprinkle each with sesame seeds and radish. Serve with Japanese hot pepper seasoning on the side if using.
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