Cut the potatoes and onion into bite sized pieces, and saute in oil.

When they are more or less cooked, add the canned tuna, oil and all, and mix together lightly.

Add sake, mirin, sugar, soy sauce, and water, and simmer while stirring. The dish is simmered in the moisture from the vegetables. If there's not enough moisture, add a little water.

When the potatoes are cooked through and full of flavor, it's done.