A few weeks ago, I ran out of Tonkatsu Sauce so I tried making my own. It is really simple and only requires four ingredients: ketchup, Worcestershire sauce, brown sugar, and oyster sauce. The combination of ingredients is surprising at first but I swear it tastes very similar to the storebought sauce.
Combine all the ingredients in a small bowl and stir until the sugar dissolves. You can use it immediately or store it in the fridge for up to three months (the sauce has a long shelf life because it's made with other shelf-stable condiments).