Crack an egg into a pint-sized wide-mouth Mason jar.
Add mustard, salt, lemon juice and white vinegar.
Add oil on top and allow it to sit for a few seconds so the ingredients separate and the oil floats on top.
Gently place your immersion blender into the jar until it's to the bottom.
Turn the immersion blender on and leave it at the bottom until the mayonnaise begins to emulsify.
Slowly bring the immersion blender up and back down, incorporating the remaining oil until all the contents are thick, creamy and completely emulsified.
Add a lid to the jar and store it in the refrigerator for up to two weeks (see my tips on storing below).