Add mirin and sake to a pot and bring to a boil. Add dashi stock, then soy sauce. Taste and add sugar as needed. Add thinly sliced onion and simmer until the onion is tender and flavorful.

Cut the drained tofu in half horizontally to make two slices. Season with salt and pepper. Coat each piece in flour, then egg, then panko breadcrumbs. Fry in oil mixed with sesame oil until golden brown.

Once the tofu is golden and crispy on both sides, remove and cut into bite-sized pieces. Lightly beat the eggs with chopsticks (about twice).

Scoop cooked rice into a bowl. Pour the donburi sauce for one serving into a small saucepan and bring to a boil. (It tastes best when made one serving at a time.)

Add the tofu katsu to the boiling sauce. Immediately pour just the egg whites over the katsu. When the whites start to set, add the yolks, cover, and steam over high heat for about 30 seconds until done.

Top with nori seaweed or green onions if you like, and enjoy!