Prepare mayonnaise: whisk yolk and mustard together. Slowly add oil while whisking continuously. Adjust salt. Set aside.

做蛋黄酱：用蛋刷将蛋黄和芥末搅匀，慢慢地加入油，并不停地搅拌，直到油被蛋黄完全吸收，加盐调味。待用。