• Gather all the ingredients. You will need a small heavy-bottomed saucepan (I use 1.5 QT); you will need to cover the eggs completely with the water.
  
• Add the water to the saucepan, cover with a tight-fitting lid, and bring it to a boil.
  
• Once boiling, remove the pot from the heat. Take the eggs out of the refrigerator. Add the cold tap water to the pot and gently submerge the cold eggs in the hot water. Immediately cover and set the timer for 17 minutes.
  
• If you'd like to enjoy the onsen tamago with the sauce, combine the dashi, mirin, and soy sauce in a small saucepan and bring it to a boil. Add the katsuobushi, turn off the heat, and let the katsuobushi sink to the bottom of the pan. After 30 seconds or so, strain through a sieve and set the sauce aside. You can keep the sauce in the refrigerator for 4-5 days.
  
• Cut the scallion into thin slices and set aside.
  
• Once 17 minutes have passed, gently take the eggs out of the water and set them aside for 5 minutes.
  
• You can enjoy the onsen tamago either warm or at room temperature. Crack the egg into a small bowl, pour the sauce into the bowl, and garnish with the sliced scallion. 
  

To Store

• You can keep the uncracked onsen tamago for 1-2 days in the refrigerator. To reheat, remove the onsen tamago from the fridge and bring it to room temperature. To warm it up further, place it in a bowl of 160ºF (70ºC) water for 10 minutes. Do not reheat higher than that; otherwise, the heat will cook the egg.