Mix eggs, Mirin, salt, soy sauce, and Benishoga OR spinach in a bowl.

Heat a rectangular Tamagoyaki pan or a small non-stick frying pan at medium high temperature and spread oil.

Pour a thin layer of egg mixture in the pan, tilting to cover the bottom of the pan. After the thin egg has set a little, gently roll the egg from one end of the pan to the other, forming a log. Start to roll when the bottom of the egg has set and there is still liquid on top. Pour some more egg mixture to again cover the bottom of the pan, with the roll of egg at one end. After the new layer has set, roll the log back onto the the cooked thin egg and roll to the other end of the pan.

Repeat adding egg to the pan and rolling to the end of the pan, back and forth, until the egg is used up.

Remove from the pan and cool for 3-4 minutes and slice into ½ inch thick pieces.