The reason why this takes no time to cook is because I use Japanese leek instead of onion.
I wanted to eat gratin potatoes, but I didn't have much time, so what to do? This is not a gratin at all but it's easy and tasty.
This tastes better if you add oil from the canned tuna. Adjust the richness of the mayonnaise sauce to taste.
Enjoy Japanese food around the world.