So-su (ソース)

So-su is the Japanese version of Worcestershire sauce, most people here in the states probably know this as "Tonkatsu Sauce" or even just "Bulldog Sauce" as a proprietary eponym (definition). I've heard that it is most similar to HP sauce or A1 Steak Sauce. I haven't tried either of those so I can't personally vouch for those claims. However, it kinda makes sense that HP sauce would be similar, because all of so-su is derived from British influence on Japanese cuisine. Japanese Worcestershire sauce is consider "youshoku" or an item of western-influenced cuisine cuisine.

Where do you usually find so-su? On top of okonomiyaki, tonkatsu, Japanese-style hamburger, yakisoba, and takoyaki, to name a few.

There's actually several different types of so-su, based on how thick it is. "Usuta so-su" is thinnest and most similar to the Worcestershire sauce that the British invent and Americans are familiar with, and the "tokuno or tonkatsu so-su" is very thick and is sweeter and less tangy than the usuta variety. The third is "chuno so-su" which is almost like a blend of the two styles: viscosity in between the other two styles, with a mix of tanginess and fruity sweetness. The type most familiar around the world would be the thickest sauce, the tonkatsu sauce.

Three styles: thin, medium, thick*

There are even specific so-su variations branded for okonomiyaki and yakisoba. A side-by-side taste test of tonkatsu sauce and okonomiyaki sauce will tell you that tonkatsu sauce has a lot more twang and bite to it, more similar to to Worcestershire sauce, while okonomiyaki sauce is much fruitier and sweet. However, I've used them interchangeably in a pinch and no one was wise to it. The ingredients themselves are very similar, but in different order (predictably, the fruit mix is higher on the ingredient list for the okonomiyaki sauce).

If you don't have access to a Japanese market, there are plenty of fascimiles you can make at home. The most common substitution is as follows:

Chuno Sauce:
1/4 cup Worchestershire Sauce (Lea & Perrins is the ubiquitous brand)

Okonomiyaki sauce

1/4 cup ketchup

Tonkatsu Sauce:
1/4 cup Worchestershire Sauce
1/2 cup ketchup

Additional easy fix: adding some sugar to HP Sauce or A1 steak sauce to make them sweeter.

However, if you're looking for something with a little more complexity, I recommend the recipes on Saveur and Serious Eats. Both of them go for soy sauce for added umami, both rely on mustard for a bit of a kick, and both deliver results.

But if you're lazy like me, and can't hoof it to a Japanese market, it's worth ordering the real deal from Amazon.

*Picture credit: Japan's Wiki on Tonkatsu Sauce.

Recipes that use So-su:
Hiroshima-Style Okonomiyaki
Holiday Ham Korroke
Julia's Korroke 
Katsu Sando
Yuzu Kosho Bloody Mary