Shrimp and Leek Soba

Name: Shrimp and Leek Soba
Ingredients: Two 4-inch-square pieces kombu (dried kelp; about 20 grams); 1 1/2 packed cups katsuobushi (dried bonito flakes; about 30 grams); 1/4 cup mirin; 1/4 cup soy sauce; 1 tablespoon rice wine vinegar; 1/2 tablespoon grated ginger; Kosher salt; 8 ounces dried soba; 4 heads baby bok choy, stems cut into bite-size slices and leafy tops kept separate; 8 ounces peeled and deveined medium shrimp, tails removed; 1 slender leek, white part only, thinly cut on a bias; 2 radishes, cut into thin sticks; 1 tablespoon sesame seeds, toasted; Shichimi togarashi (Japanese seven-spice mix), optional