Chutoro is the part of the tuna that isn’t as fatty as otoro, but not as lean as the reddest part of the tuna. It’s halfway, but is also part of the tuna belly.
When you get a chunk of tuna that includes all three levels of fattiness, cut off the fattest part (the palest), then cut off the reddest part. What remains is chutoro.
Cut the chutoro into long rectangular fillets, 1 inch thick. Then, cut diagonally into 1/8 inch (5mm) slices. Do this by slanting the knife tot he right and slice from the left end.
You will see that pieces will be triangular. To make it a rectangle, simply make a slit into the fish starting from the wide part and open it out to form the rectangle.
It is now ready to make into nigiri sushi, or serve as is for sashimi.