Cooked
this with my brother this evening. Granted, he ended up taking over a lot of the cooking and I helped get things and cleanup around him. Some of that was interrupted by a weird dizzy spell.
He made the sauce for that from scratch and had to improvise a lot of stuff. He made a clam sauce in the absence of oyster sauce, and in the absence of ketchup...made an AMAZING ketchup out of tomato paste, vinegar and sugar. Which kicked me out of my dizzy spell with its flavor.
The resulting fake Tonkatsu sauce is pretty great.
As for the actual korokke...it's not done yet.