First place oil in your immersion blender container or a wide mouth pint mason jar.
Next add in the egg, lemon juice, salt & mustard.
Gently place the immersion blender at the bottom of the container, over the egg yolk.
Start blending, holding the immersion blender in place until it starts to emulsify.
Slowly raise the immersion blender up the container, bringing it down if needed, until it is fully emulsified. (Do not over mix or the mayo will separate.)