Tonkatsu Sauce とんかつソース
In every country there are some best selling instant sauce. Heinz ketchup, Magie bouillon, etc. I suppose that the Japanese food maker or cook must have learned a lot from British worcester sauce. In Japan it is called Ustah-Sauce. Japanese people look like fond of thick sauce. They changed Indian curry sauce thick, British worcester sauce thick. There, the big hit "tonkatsu-sauce" was born. You can try Japanese worcester sauce, middle thick sauce or thick tonkatsu-sauce according to your favourite.
