Make the dashi by adding the water and dashi packet to a small pot. Bring to a boil over medium heat, then reduce to medium-low and simmer for 2 minutes. Discard the packet.

Thinly slice the onion and green onion. Crack the eggs into a bowl and use chopsticks to lift the whites 5-6 times to break them into clumps without fully whisking.

Place onion slices in a single layer in a large frying pan. Add the dashi and cover with a lid. Heat to medium until the onion is tender and almost translucent.

Add the sugar, mirin, and soy sauce to the pan and mix together. Place the sliced tonkatsu cutlets on top of the onions, cover, and heat until the broth is simmering, about 2 minutes.

Evenly distribute the eggs in a circular pattern over the tonkatsu and onions. Cover and simmer for about 1 minute until the egg is just barely set.

Divide the rice into large donburi bowls. Slide the tonkatsu, onion, and egg mixture on top. Drizzle with extra broth and garnish with sliced green onion.