A rice bowl topped with katsu, a deep-fried pork cutlet.
The most common recipe uses a flavoring sauce, onions and beaten eggs. However, there are other variations, including one well-known recipe which uses Worcestershire sauce and shredded cabbage. To differentiate the two, the former is sometimes called "Tamago-toji [bound-together-with-egg] katsudon," whereas the latter is called "Sauce Katsudon."
A common trope in Japanese crime dramas is for the detective to eat katsudon while interrogating a suspect. The typical heart-of-gold type detective would think, "if this guy goes to the slammer, he'd never be able to eat anything as good as this again," and then use his own coin to order a katsudon delivered straight to the suspect's house. Then the suspect would be moved so much as to turn himself in and confess to the crime, tears streaming down his face. And so goes the cliche. For these stories, there is an explanation that katsudon was the most expensive food that one could get on a police officer's salary.
This is nowadays only seen as a joke or stereotype, but Japanese people have come up with the expression, "let someone eat katsudon," which carries the same meaning as "read someone their Miranda rights" (see post #1184509).
