The egg has a unique texture in that the white tastes like a delicate custard (milky and soft) and the yolk comes out firm, but retains the colour and creamy texture of an uncooked yolk. This special texture is the result of the egg yolk and egg white solidifying at different temperatures.
The egg is poached within the shell and is served with the shell removed in a small cup together with a sauce of broth and soy sauce.
The traditional way of cooking onsen tamago is to place eggs into rope nets and leave them in an onsen, with water that is approximately 70 °C (158 °F) for 30 to 40 minutes. After cooking, the shell is cracked open and the egg is served in seasoned bonito dashi (Japanese stock) for breakfast, or with a light sauce made with mirin, dashi and soy sauce with chopped spring onions sprinkled over the top.