In a tall jar or bowl, add the oil, egg, lemon juice, Dijon mustard, and salt.
Insert an immersion blender into the jar, placing it at the bottom, ensuring the egg is covered. Turn it on and blend without moving it for 10-15 seconds until the mixture begins to emulsify.
Slowly lift and tilt the blender to incorporate the remaining oil until the mayo is thick and creamy.
Taste your mayo and add more salt or lemon juice, if needed, for your desired flavor.