Soup

The logical choice of soup after sashimi would certainly be miso shiru (味噌汁).

Don't look down upon this simple brownish liquid. It has quite a bit of science.

Miso shiru has three major ingredients: miso (味噌/麵豉), konbu (昆布), and katsuobushi (鰹節).

Miso is made of soy bean, and represents a great source of vegetable protein, sodium, and various vitamins (including B12).

Konbu is a rich natural source of glutamic acid. Yes, with sodium, it becomes monosodium glutamate - that's why it adds flavour to the soup. (Ancient Japanese had the wit to call this excellent but poorly defined taste umami [旨味]. Alas, this is what we now know as 鮮味.)

Katsuobushi also stimulates our taste buds - by its rich content of inosinic acid. It is, in other words, a purine derivative, and the building block of nucleotides.

And, with all these three, we have an almost ideal oral nutritional supplement - in a solution !

PS. An excellent property of miso shiru is that you can add whatever kind of ingredient to it. To show their support to environment protection, our G8 leaders should use the bones and leftovers of those fishes used for sashimi to make the base of the soup. It adds to the flavour, and it was the way how many poor ancient Japanese could get some animal protein.