The egg yolks can be firm, soft or completely runny. What is important is that whites are firm. This prevents the boiled eggs from splitting during shelling and soaking.
Mix together the soy sauce, mirin and rice vinegar in a bowl.
Drop the shelled eggs into the mixture and leave for about 15 minutes. If the eggs are not completely submerged, use a teaspoon to roll them around every few minutes.
Drain the eggs, cut into halves if you prefer and serve.