Cut Tonkatsu into strips, set aside.
In a small frying pan, add Dashi, soy sauce, sugar, Sake, and Mirin, and cook at medium heat until it boils. Reduce heat to low-medium.
Add cut Tonkatsu to the sauce in the pan. Beat egg(s) in a small bowl and pour over the Tonkatsu. Cover and cook for 1 minute. Sprinkle chopped green onions if you like.
Slide Tonkatsu and egg(s) with sauce over rice in a bowl.
Sprinkle sliced Nori on top.