Kojis Sushi & Shabu Shabu at The Block at Orange makes ‘beef crêpes’. (That’s what they call them. I know there’s a Japanese name for them.) Cut up like a sushi roll, they are served with rice and a salad. It also has a sauce that you can dip the meat into, as well as use it as salad dressing. I asked once, and was told that it was basically mayonnaise and soy sauce.
I’ve just tried to imitate the sauce, using unmeasured quantities of Best Foods mayonnaise, Kikkoman soy sauce, and rice vinegar. It came out pretty well, actually. I used it on some ahi instead of ‘beef crêpes’, so I don’t know if it’s quite right. Not that I would remember, since I moved up here six years ago.
But I think it should be available in most asian supermarkets.
On the other hand, if it was indeed more watery, like a normal soy sauce it might be a sauce called Ponzu (sweetened soy sauce with a tang), which you should also be able to find in an asian deli. Here’s a pic of my favorite Ponzu
Yes, savoury and thick; thick like French dressing. No catsup in it though. What I made tonight tasted pretty close, as well as I could remember. Maybe it’s something that Koji’s came up with, rather than a traditional sauce? (‘Japan’ and ‘mayonnaise’ don’t really go together in my mind.)
You might have better luck starting with Japanese mayonnaise if you can’t find a concrete recipe.
Kewpie is probably the most popular brand, and it’s available online and in Asian markets. If you don’t want to buy it, consider that the main differences are in the vinegar (generally speaking, Japanese mayo uses rice wine vinegar rather than white vinegar) and whether or not there’s MSG (not so much for US mayo, more likely for Japanese).