Help With Replicating a Dish

Hello r/Japanesefood . I was in Las Vegas recently and ate at Nobu in the Paris casino. I ordered the Beef Tenderloin with Yuzu Truffle Butter and it was one of the best things I've ever eaten. I have since then tried to research and find a similar dish to replicate it to no avail. The onions that the steak was served on were caramelized and very sweet.

I've bought Yuzu juice and Yuzu Koshi and tried them a few different ways including making a butter but I've gotten nowhere. Does any have any suggestions or has made something similar?

Thank you all.