Crack eggs into a bowl and add dashi powder, mirin, and kosher salt. Whisk until fully combined.
Add finely chopped green onion, carrot, and onion to the egg mixture. Stir to combine.
Heat a 9-inch nonstick pan over medium-low heat. Add a small amount of oil and spread with a paper towel.
Pour a thin layer of the egg mixture into the pan, tilting to spread evenly.
When the egg is 80% cooked, start rolling it gently toward one side using chopsticks or a spatula.
Move the rolled egg back to the center of the pan. Oil the pan again lightly.
Repeat the process: pour more egg mixture, let it cook, then roll again. Continue until all the mixture is used.
Once done, remove the roll from the pan and let it rest for 2 minutes before slicing into even pieces.