The cook time specified below presumes that you have already cooked your tonkatsu.We like the eggs to be just cooked for our donburi dishes. Soft and barely set. If you prefer them to be firmer and more thoroughly cooked, cook them for 30 seconds longer.
In a wide shallow pan, boil the mirin, sake, soy sauce, sugar, dashi, sliced onion and half of the scallions over medium heat. Cook until the onion slices are softened, about three minutes.
Pour the beaten eggs over the broth.
Lay the sliced tonkatsu on top and boil for about half a minute to allow the eggs to stick to the bottom and sides of the pork as they cook.
Slide the contents of the pan over rice in a bowl.
Sprinkle with the remaining scallions (and sesame seeds, optionally) and serve your katsudon immediately.