Pour 5 cups (1200 ml) of water into a pot and bring to a simmer. Cut the kombu and soak it in the water for 5 minutes.

Remove the kombu, then add the bonito dashi packet and let it steep for 4 minutes.

Slice the kamaboko into 1/4-inch (5 mm) pieces and cut the napa cabbage into 2-inch (5 cm) pieces. Add both to the pot.

Remove the bonito dashi packet, then add the kombu (cut into bite-sized pieces) back into the pot.

Add the mirin, soy sauce, and sake to the pot.

Fillet the ayu into three pieces each, sprinkle with salt, dip in a batter made from tempura flour and ice water, and deep-fry.

Tear the maitake mushrooms into large pieces, dip in the tempura batter, and deep-fry. Since they absorb oil quickly, remove them from the oil promptly.

Boil the udon noodles. Pour the soup into bowls, add the noodles, and top with the fried ayu and maitake tempura.