Tonkatsu Sauce is a Japanese sweet sauce served to accompany Tonkatsu.
Recipes to make your own generally call for ketchup, rice wine, soy sauce, Worcestershire sauce, sugar, rice wine vinegar, mustard powder, allspice, cloves and garlic powder.
You can buy it ready-made in stores in bottles; “Bulldog” is a popular brand.
Substitutes
Ketchup mixed with steak sauce or Worcestershire sauce.