In a mixing bowl, whisk together eggs, dashi stock, sugar, soy sauce, and mirin until well combined.

Heat a rectangular tamagoyaki pan (or small non-stick skillet) over medium-low heat. Lightly oil the pan using a paper towel.

Pour a thin layer of egg mixture into the pan, tilting it to coat the surface evenly. When the egg is set but still slightly soft, roll it to one side of the pan using chopsticks or a spatula.

Add another thin layer of egg mixture to the pan, lifting the rolled omelette to let the raw egg flow underneath. Cook and roll the omelette again. Repeat this process until all the egg mixture is used.

Remove the omelette from the pan and shape it into a neat rectangle using a bamboo sushi mat (optional). Let it cool slightly before slicing into bite-sized pieces.