Genghis Cohen is a Chinese restaurant/lounge in Los Angeles, and it appears that Beck jammed with friends as a surprise party for Greg Kurstin's birthday.
One instagram post, by singer CC White, mentioned "All star jamming with The Bird And The Bee, Jackson Browne, Beck and more." Beck's bassist, Justin Meldal-Johnsen, similarly posted a picture saying that he and Dave Grohl were the rhythm section for the evening.
I have seen no evidence of what they played or anything; I doubt anyone filmed it, it was just for fun. Probably lots of David Bowie though haha.
[people and bands
Backup Band: Beck Hansen, Billy Bush, CC White, Greg Kurstin, Inara George, Jackson Browne, Rami Jaffee
Bass: Justin Meldal-Johnsen
Drums: Dave Grohl