Slice the fish into long strips of roughly the same width and height. This will improve the ‘look’ of the sear on the slices.
Season the raw tuna generously with salt, pepper, and sesame seeds on both sides.
Melt the butter into a super-hot skillet (we use a cast iron skillet for this). When it starts to brown, add the tuna and sear for one minute on the top & bottom, plus 15 seconds on each of the remaining two (long) sides. The time should be two minutes and thirty seconds total.
Sear all the way around the outside of the tuna, while keeping the interior rare (not raw). In rare tuna, the thin ‘striations’ of muscle become more pronounced and white, as you see in the photos.
Once the tuna is seared on all sides, place on a dish and reserve.