In a pot, combine soy sauce, mirin, and sake
Let it come to a boil for about a minute and remove from heat
In a separate bowl pour sauce and place a piece of kombu with it
Place sauce in the fridge or freezer to cool the sauce. The sauce should be chilled entirely before use
While the sauce is cooling down, slice tuna sashimi
Once the sauce is chilled, place tuna slices in the sauce
In the fridge, let it marinate in the sauce for 1-2 hours. Keep it in longer if you like a stronger taste; however, I don't recommend longer than overnight
In a bowl, place cooked rice
Add marinated maguro sashimi pieces on top
Top with toppings of choice
Serve and enjoy