Before You Start
Gather all the ingredients.
For the steamed rice, please note that 1½ cups (2 rice cooker cups, 300g) of uncooked Japanese short-grain rice yield 4⅓ cups (660g) of cooked rice.
This is enough for 2 donburi servings (3⅓ cups, 500g).
To Make the Dashi
Add 1 cup water and 1 dashi packet to a small pot.
Start cooking over medium heat.
After it starts boiling, reduce the heat to medium-low heat and simmer for 2 minutes.
Shake the bag a few times to release more flavor and then discard the packet.
The dashi is ready to use.
To Prepare the Ingredients
Thinly slice ½ onion; cutting it thinly reduces the cooking time.
Slice 1 green onion/scallion and set aside for garnish.
Crack 3 large eggs into a medium bowl.
Lift the egg whites 5–6 times with a pair of chopsticks to "cut" them into smaller clumps.
This will marble the yolks and whites and prevent the whites from falling into the frying pan all at once.
Do not whisk or beat the eggs.
Cut 2 tonkatsu cutlets into slices about 1 inch (2.5 cm) thick.
To Cook the Katsudon
Place the onion slices in a single layer in a large frying pan that can fit 2 cutlets.
Add the dashi to the pan.
Cover with a tight-fitting lid to prevent evaporation.
Turn on the heat to medium and cook until the onion is tender and almost translucent.
Add 2 tsp sugar and 2 Tbsp mirin.
Add 2 Tbsp soy sauce and mix together.
Place the 2 tonkatsu cutlets on top of the onion and cover with the lid until the tonkatsu is heated through and the broth is simmering, about 2 minutes.
The bottom of the tonkatsu will absorb the broth.
Evenly distribute the egg in a circular (spiral) pattern over the tonkatsu and onion.
Avoid the edges of the pan where the egg can easily overcook.
Cover and simmer for 1 minute, or until the egg is just barely set.
To Serve
Divide 2 servings cooked Japanese short-grain rice into large donburi bowls.
Place the tonkatsu, onion, and egg mixture on top.
Drizzle with extra broth, if you'd like.
Garnish with sliced green onion and serve immediately.