Rayu is a spicy red oil that is never far at hand at ramen shops or anywhere that the fried or steamed dumplings called gyoza are served.

In the last year or so, though, the condiment (sometimes seen written as la-yu) has taken center stage in a chunkier, more flavorful incarnation that has gone from foodie indulgence to the fast-food mainstream. Taberu, or edible, rayu has the same chili-infused sesame oil base as the pourable stuff, but it’s packed with minced chunks of fried garlic and onion that give it a chunky, spoonable consistency.  Momoya released jars of it one year ago with a mouthful of a name that translates to “It looks spicy but isn’t too spicy but is a bit spicy” last year. When it sold out, S&B Foods released a not-so-subtle copycat version called "Pour it on! Main dish rayu, a bit spicy." Now it’s gone well beyond grocery store shelves and is turning up as a potato chip flavor, a tonkatsu topping, and most recently, glopped on the patties at Japan’s homegrown burger joint Mos Burger.

The Mos Burger concoction, which had people lining up outside an Akihabara location on its first day, is a “collaboration” with morning TV show "Sukkiri!" star Terry Itou.  The full name of the burger is "Terry Itou’s crunchy rayu burger." Mos sold some 2.1 million of them, twice as many as anticipated in the original sale period of July 10 to 28. For those who missed out the first time around, they’re going to serve up another million of the spicy burgers at ¥390 for a regular burger and ¥420 for one with cheese. (The cheese, incidentally was recommended in one review to cut the spice for people who found it too hot.)