Put the chicken legs in a 5- to 8-quart slow cooker, and crumble the miso on top. Add the scallions, garlic cloves, shiitake mushrooms (if using) and 6 cups water. Stir well to combine. Cook until the chicken is tender, at least 4 hours and up to 6 hours on low. If it’s more convenient, you can let the slow cooker switch to warm after 6 hours. The soup will hold on warm for about another 2 hours before the chicken begins to dry out.

Switch the heat to high. With a slotted spoon, remove the chicken, scallions, garlic and shiitakes, and place in a bowl. Set aside to cool. Stir in kombu, bok choy, tamari and mirin. Cover and let cook until the bok choy is wilted and tender, 5 to 10 minutes. Remove and discard the kombu. Coarsely shred chicken meat into the soup, discarding the skin, bone, scallions, garlic and shiitakes. Taste the soup and whisk in a few more spoonfuls of miso or tamari, if desired.

Divide the noodles among 4 or 5 bowls, and ladle the soup on top. Top each with sliced scallion, a halved soft boiled egg, sesame seeds and a piece of nori.