The latest article in Felicity Cloake's very useful “How To Make Perfect...” series in the Guardian concerns mayonnaise. Online, you get readers' comments as well.
In Cloake's own recipe, she adds oil to egg yolks and salt. I have always mixed Dijon mustard with my egg yolk(s) first. (And I have used a pestle and mortar, which have never given me as much grief as she seems to experience with them.) But yesterday I found that I had run out of mustard, so I tried her method. I ended up with a thin yellow liquid - my first ever failure.
The mustard - I generally use half a teaspoon of it with one yolk - helps to emulsify the sauce. As soon as the oil is incorporated with the yolk, the emulsification occurs. But that was not the case when I carried out my sorry experiment. The oil and yolk combined but did not thicken - perhaps I should have used a whisk. Nevertheless, I carried on, rather as if I were in a hole and persisting to use a spade.
I never use a blender. Blades damage the flavour of oil, in my view.
Here is my mayonnaise recipe. Nowadays, I use a ratio of sunflower/groundnut oil to olive oil of 125g/25g.