Special equipment: cheesecloth and a fine-mesh strainer
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.