To make the marinade, combine mirin and sake in a saucepan, bring to a simmer and then add soy sauce, bring it to boil and immediately turn off the heat. Let it cool.
Soak the fish in the marinade for 15~20 mins.
Arrange the rice in a bowl and place the shiso leaves over the rice, top with the marinated sashimi slices. Pour the sauce over the rice and sashimi. Garnish with the wasabi paste and grated ginger.