Slice the chicken breast diagonally into pieces about 1/2 inch (1.5 cm) thick. Season with salt and pepper. Dredge each piece in flour, then dip in beaten egg, and coat with panko breadcrumbs. Fry in oil at 340–355°F (170–180°C) until golden and crispy.

Thinly slice the onion and cut the green onion (scallion) diagonally into thin strips. In a skillet, combine the dashi stock, sugar, mirin, soy sauce, and sliced onion. Cover and cook over medium heat until the onion is translucent and lightly colored by the soy sauce.

Spoon cooked rice into bowls. Cut the chicken katsu into bite-sized pieces and place the desired amount on top of the rice.

Once the onion is cooked, turn the heat to high and add the green onion. Lightly beat the eggs and pour them evenly over the skillet. Gently stir the eggs around the pan 2–3 times with chopsticks, then immediately reduce to medium heat. Cover and count to 10 seconds, then turn off the heat. Let it steam for 1 minute, then uncover and spoon the egg mixture over the rice bowls.

Top with chopped nori and serve!