1cupoilideally grapeseed oil, if not sunflower oil
salt and pepper
In a round bottom bowl, first mix the egg yolk and the mustard. Add salt and pepper.
Then gradually pour the oil in a very thin and regular dash and keep stirring vigorously with a whisk. Make sure you don't drop the oil too quickly - if so stop adding oil and stir even more vigurously to incorporate the excess of oil - and don't stop stirring and pouring oil until you don't get a smooth aerated texture.