Since you’ll be eating raw fish, it is very important to get your hands on the highest-quality yellowtail or white tuna possible. Fresh fish should look bright, shiny, and translucent. I always recommend sourcing the fish from the best seafood market available. If you live in San Diego, the Tuna Harbor Dockside Market is one of the best location to get fresh seafood.
First off, try not to break the cold chain. Place a couple of clean baking trays in the freezer before going to the fish market. When back home, always remove fish from its original packaging, rinse under cold water if necessary. Pat dry and wrap fish in plastic cling or parchment paper. Grab one frozen tray from the freezer, top with ice packs and lay fish over. If you don't have ice packs, sub it for a sealed bag of ice. Replace ice packs every day. Keep in mind that freshly caught fish can be stored in the refrigerator for up to 6 days. Otherwise 48 hours.
Trim off any remaining blood line and discard. Separate the meat from the sinew when skinning the loin. With a large spoon, scrape flesh out left on the skin and save later for tartare. Cut the loin into 2 or 3 portions and slice out into desired thickness. Keep fish on ice until ready to serve.
What makes this dish so enjoyable is the addition of truffle oil. Needless to say that using real white or black truffle shavings would bring this dish to another dimension.
Peel, trim and cut potatoes into the same thickness as you did for the fish. Overlap potato slices with the fish. Season generously with fleur de sel, pepper, truffle oil, vinegar and scallions.. Enjoy!