Combine the base

• Place the egg yolk, mustard, vinegar and salt in a tall narrow bowl.

Emulsify the mayonnaise

• Pour in the oil and let it settle on top of the yolk mixture.
• Place the immersion blender at the bottom of the bowl and blend without moving it until the emulsion starts to form.
• Slowly lift the blender up and down to incorporate the remaining oil until thick and glossy.

Adjust and store

• Taste and adjust the salt and vinegar if needed.
• Transfer to a clean jar and refrigerate.