Combine the dashi, usukuchi, mirin and kombu in a pot. Bring to a boil over high heat. Add the salt and katsuobushi, and immediately turn off the heat and strain into a nabe (hot pot) or medium saucepan. Add the udon and bring back to a boil. Reduce the heat and add the chicken and shitake mushrooms. Cook for 1 minute.
Crack the egg into center of soup. Ladle the soup over the egg until it turns white. Remove the nabe from the heat and add the kamaboko and blanched spinach.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.