The star of the dish is of course the karashi mentaiko (marinated spicy pollock roe) or known as myeongranjeot in Korean. Though it was only introduced to Japan over a century ago, it is now considered a common ingredient in Japanese cooking. It is difficult to describe the taste of mentaiko; it is salty, savoury, fishy , a little spicy and utterly delicious! ... words failed me really so do make it one of the ingredients you must try before you die.
P.S My dear friend A had started a lunch group at work with 5 of her colleagues (each contribute a fixed amount of money and each would take turn making lunch etc etc.) and I was quick to say yes when she asked me for my assistance when it was her turn to cook. This formed one of the components of her bento boxes.