Equipment: 

• whisk
• medium sized heat resistant bowl (glass of metal)
• 2 medium sized pots

Method:

1. Let's make the castric. Combine vinegar and water with the onion and bayleaf in a medium sized pot. Bring to a boil and let cook for 5 minutes on medium heat. Let cool afterwards.
2. Take the second pot and fill half way with water. Bring to a boil. Once the water is boiling place the heat resistant bowl on the pot and add the egg yolks, as well as 20 ml of the castric. Directly start to beat the egg yolks with the whisk until the egg yolk is cooked (it should nearly double in size and become thicker). This technique is called "Au Bain Marie". 
3. Continue beating the egg yolk and add the lemon juice, salt and the liquid butter. Mix well. 
4. Take the sauce off the heat and season for taste with salt.

Tip: Best to be served warm.