Sashimi is one of the things that YELLS GET THE BEST INGREDIENTS and then process it right. (READ: catch) the best fish, then processit correctly – that involves bleeding it & allowing it, at least for yellowtail, to go through rigor mortis to break down the cell structure and in turn, make the flesh more tender. I’ve had yellowtail one minute after it’s been killed and prefer it 2-3 days after because it melts like butter in your mouth. Different fish will require more or less time. Bonito sashimi, for instance, IMO is only good day of.
After you get the fish, it’s important to keep it COLD. It’s always better to keep the fish on the bone until you’re ready to eat it (think of an avocado pit + avocado – keeps it greener), and when you’re ready, dismantle the loin from the belly section. Or just goonie goo goo the damn thing. 
slice across the grain
then proceed with the next ones…
stacks on stacks! Decided to cut some with more blood line, and some with less. The white is the belly section. 
yellowtail plate 🙂