To the heavy-bottomed saucepan, add 4¼ cups water (1 L). Cover with a tight-fitting lid and bring it to a boil.
Once boiling, turn off the heat and remove the pot from the stove. Add ¾ cup tap water (200 ml) that's colder than room temperature and stir.
Gently submerge the cold eggs, one at a time, into the hot water using a fine-mesh skimmer. Immediately cover with the lid and set a timer for 17–20 minutes.
Meanwhile, make the sauce by combining ¼ cup dashi, ½ Tbsp mirin, 1½ Tbsp soy sauce, and 2 Tbsp katsuobushi in a small saucepan and bring it to a boil. Once boiling, turn off the heat and strain through a sieve.
After 17–20 minutes, gently take the eggs out of the water, one at a time. Set them aside for 5 minutes.
Cut the green onion/scallion into thin slices.
Enjoy the Onsen Tamago either warm or at room temperature with the dashi-based sauce and sliced green onion.
