Gather all the ingredients. I use a dashi packet method to make dashi (Japanese soup stock) today. Other choices are to make dashi from scratch using kombu and katsuobushi, or to use a convenient dashi powder. Please read my ultimate dashi guide if you're new.
To Make Dashi
In a small pot, add water and a dashi packet. Start cooking over medium heat.
After boiling, reduce the heat to medium-low heat and simmer for 2 minutes. Shake the bag a few times to get more flavors out. Discard the packet and dashi is ready to use.
To Prepare Ingredients
Thinly slice the onion and green onion. I highly recommend cutting the onion thin to reduce the cooking time.
Beat the eggs in a medium bowl.
Cut the tonkatsu into 1 inch (2.5 cm) thick pieces.
To cook Katsudon
In a large frying pan that can fit 2 tonkatsu pieces, place the onion slices in a single layer and add dashi.
Cover the frying pan with a tight-fitting lid (so the broth does not evaporate too quickly). Start cooking on medium heat until the onion is tender and almost translucent.
Add the sugar and mirin.
Add soy sauce and mix together.
Place tonkatsu over the onion and cover with the lid to reheat the tonkatsu, about 2 minutes. The bottom of the tonkatsu will absorb the broth.
Once tonkatsu is reheated, slowly drizzle the eggs over the tonkatsu and broth. If you don't like the raw green onion taste, you can add it over the egg mixture now. Cover with the lid for 1 minute, or until the egg is just set.
Serve steamed rice in large donburi bowls and place the tonkatsu, onion, and egg mixture on top. Drizzle extra broth if you like. Garnish with green onion and serve immediately.
You can keep the leftovers in an airtight container and store them in the refrigerator for up to 3-4 days.