To make mayonnaise you start with egg yolks and mustard (and maybe other seasonings): the mustard helps the mixture to emulsify. You can put more mustard in it,

To make mayonnaise you start with egg yolks and mustard (and maybe other seasonings): the mustard helps the mixture to emulsify. You can put more mustard in it, to make a mustard mayonnaise, or add more mustard later.