At most Asian shops you'll find store-bought tonkatsu sauce, but this sauce is super easy to make with only a few ingredients. An amazing Japanese sauce.
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If you can't find any Worcestershire sauce, you can replace it with a mixture of 1 teaspoon of sugar, 1 tablespoon of salty soy sauce, and half a teaspoon of lemon juice.
Mix the ketchup with soy sauce, Worcestershire sauce, sugar and soy sauce. Mix this well until the sugar is slightly dissolved. There is no need to heat this simple sauce.