Heat water in a small pot over medium heat. Bring to a slow boil.
Reduce to lowest heat and bring the water to gentle simmer
In a heatproof mixing bowl, add in egg yolks, mentaiko and water (or lemon juice) and place it over the pot of simmering water. Ensure that the bottom of the mixing bowl is not touching the water
Whisk the egg mixture vigorously and continuously. Continue to whisk until the egg mixture thicken and double its volume.
Slowly add in melted butter and whisk vigorously while adding
Once the yolk mixture and butter in combine, remove from the heaf.
Add salt and pepper if necessary Do note that mentaiko is quite salty. Taste before adding.
Use as require.
Recipe by eckitchensg at http://eckitchensg.com/2015/10/unami-rich-mentaiko-hollandaise/