We got a 2 lb of block of fresh tuna, sashimi grade, from Catalina Offshore Products (2 lb is the minimum you can order for sashimi-grade fresh tuna). It has ootoro 大トロ, chutoro 中トロ, and a portion called ‘chiai” 血合い. You have to first remove the skin and chiai, and then, block out these portions to rectangular blocks called ‘saku” 冊 from which sashimi pieces can be cut. In other words, you have to know how to separate these portions to prepare the tuna block from Catalina. After I make the sashimi blocks, I wrap it using kitchen parchment paper, then using a paper towel. I put them in a Ziploc bag and put them back in the Styrofoam container in which the tuna came, add more ice gel packages, which I keep in the freezer, on the top. I put the entire Styrofoam box in our spare refrigerator. This appears to be the best way to keep these sashimi and the ice gels will not totally melt for 5 days or more. I can safely keep them up to 3 days. You could get frozen toro instead, which is easier to prepare since only toro is included, although you have to thaw it. We also tried the frozen toro and the quality is very good.
I cut the chiai portion into small (1/2 to 1/3 inch) cubes and marinated in soy sauce, sake, and mirin mixture (2:1:1). You could add grated ginger (which I did not). I got enough chiai from the 2 lb block for two servings (probably about 100 grams or a bit less than 1/4 lb). I marinate over night in the refrigerator.