Whisk together egg, soy sauce, mirin, sugar in a mixing bowl (I am using a measuring cup because it is easier to pour)
On low heat, oil pan and remove excess oil with paper towel.
Pour a thin layer of the egg mixture into a rectangle shaped tamagoyaki pan. If you do not have a tamagoyaki pan, you can use a regular pan
Once it is half cooked, gather the egg onto one side of the pan
Pour a thin layer of the egg mixture, lift the cooked egg so the runny egg will steep under the cooked egg. This ensures a smooth transition and gives it the rolled effect
Once the egg mixture is half cooked, roll the omelet over to the other side
Continue this process until the egg mixture is gone
Make sure to shape and press into a rectangular shape as you continue to build the omelet
Remove from heat and chill for 10 minutes before slicing
Slice in ½ in pieces and it is ready to be packed in a bento