Put the egg yolk and mustard in a bowl. Mix briefly with the whisks of the mixer. Instead of the mixer you can also mix the mayonnaise with an egg whisk. With this method, you will notice when the mayonnaise has the right consistency.
Stir in about 1⁄5 of the oil drop by drop until the oil and egg yolk have combined to form a smooth cream.
Then slowly add the remaining oil in a thin stream, stirring continuously, and continue beating until a thick, creamy mayonnaise is obtained.
Finally, stir 1 tablespoon of lemon juice into the mayonnaise. Season with salt, pepper, 1 pinch of sugar and possibly lemon juice.
TIP: Stir in 1 tablespoon of lukewarm water at the very end to preserve the stability of the mayonnaise.