Store in the fridge for up to 1 week. You will notice the mayo will start to separate as it gets old.
Using a pint mason jar and an immersion blender, pour oil into the jar first.
Add egg then lemon juice, mustard, and salt.
Place immersion blender into the jar and rest on the bottom of the glass. Start pulsing the blender until you notice the oil starting to turn creamy.
As it starts to become creamy, start raising the blender up as you blend. Once you blend to the top, move the blender up and down until you reach the consistency of the creamy mayo and the oil is fully combined.