(Alternatively, you can make just make thin layers of scrambled eggs seasoned with a touch of salt.)
(For the tamagoyaki) In a large measuring cup, add the eggs, mirin, salt, soy sauce, and sugar. If using cornstarch, mix it with water in a separate bowl first to dissolve all the cornstarch, then add this slurry to the cup with the eggs. Otherwise, just add the water to the eggs. Beat thoroughly until well-mixed.
1/4 tsp salt, 1 tsp soy sauce, 1 tbsp sugar, 1/4 cup water, 1 tsp cornstarch, 1 tbsp mirin
Heat up a rectangular tamagoyaki pan or round nonstick skillet on low-medium heat. Once warm, brush a thin layer of oil.
Neutral oil for cooking
Give the eggs a stir, then add enough to just cover the bottom of the pan in a thin layer. Let it cook until it sets — if it browns too quickly, turn down the heat.
Once the egg layer is about 80% cooked (the surface should still be a bit shiny), use a spatula to carefully roll the egg onto itself towards the edge of the pan.
Brush the pan with more oil, then repeat with more of the egg mixture. Make sure to lift the rolled omelette to allow the egg mixture to spread under it. This will allow this new egg layer to stick to the omelette.
Once the new layer is about 80% cooked, use a spatula to carefully roll the egg (starting from the end with the previously rolled omelette) and roll it towards the edge of the pan.
Repeat until you’ve run out of the egg mixture.