Tonkatsu sauce is made from fruit and vegetable purees such as apple and tomato puree, sugar, salt, spices, starch and caramel. The ingredients are similar to Western Worcestershire but the consistency and taste are different.
Tonkatsu sauce is used on a wide variety of foods such as:
Tonkatsu (Japanese deep fried pork)
Okonomiyaki (Japanese omlettes)
Takoyaki (Japanese deep fried octopus balls)
Kushikatsu (Japanese deep fried kabobs)
Yakisoba (Japanese fried noodles)
Korokke (Japanese croquette)
Since it is commonly found on restaurant tables in Japan people put it on most anything. I saw a school girl in Tokyo put it on Lasagna. I got curious and tried it a few days later and it was not bad.