Slice the negi and leave in a bowl of cold water until step 3, then strain.Thinly slice the yuzu zest, and cut 4 slices of kamaboko (if using).Cut the chicken breast into bite-sized pieces.
Put the soup ingredients in a pot over medium-high and bring to the boil. Add the chicken and boil for 3 minutes or until cooked through, removing any scum that may appear at the surface of the soup.
Boil the soba according to package instructions, then drain and divide into two bowls.
Pour the soup liquid into the bowls, add the chicken and strained negi, and top with the kamaboko and yuzu zest (if using).