Combine sesame oil, soy sauce, teriyaki, garlic salt, and pepper in a ziplock bag. Marinate the tuna for at least an hour.

Prepare precut clingfilm on the side near you, as this all has to move pretty fast. In a nonstick high-quality pan, drizzle more sesame oil and heat over medium-high heat. When hot, place the tuna pieces in. After a minute or two (when you see the sides start searing and turning white), flip and do the same on all three sides.

When seared on all sides, immediately remove from pan and place each tuna log on a piece of clingfilm, wrapping each one tightly into a cylinder by rolling it and then twisting both sides of clingfilm tightly so it almost looks like a giant candy in a wrapper.

Freeze the wrapped tuna pieces.

Before serving, take out from the freezer desired amount and let it slightly defrost (about an hour), so that you can cut it without it falling apart.

Remove clingfilm and cut into thin slices with a very sharp knife when still quite frozen, and this way you can achieve really thin slices.