I just finished polishing off a plate of "iwashi" and "tsukudani," and frankly, I think I deserve a medal. Why? It took me over a decade to accomplish this feat! Iwashi is sardines. Tsukudani is, well, no one really knows. It's black, it kinda glistens, and has a dark but sweet taste. Even my dictionary, which lists all kinds of fish and Japanese foods, avoids a description of tsukudani. My neighbor Kazuko says it's sometimes, but not always, a type of seaweed boiled down for three hours in a mixture of soy sauce and sweet "mirin."
Why anyone ever decided to combine the tastes of sardines and tsukudani into one dish is beyond me. It's like saying, "You know what would go great with this whale meat? Graham crackers!" Iwashi is also said to go well with rice, but definitely not bread.
I've never understood how they decide on the various food combinations in Japan, but if it's anything like the other Japanese arts, culinary combining probably entails years of training and reflection, which mostly takes part through high membership fees in professional organizations such as The Society of Food Combiners. In this society, experienced, graying chefs instruct young apprentices to memorize a prescribed list of the only permissible combinations of Japanese food. This is in preparation for a test, of which there are different certified levels, with true/false questions such as "iwashi is to tsukudani as whale meat is to graham crackers."