In this exact order, add the egg white, lemon juice, salt, mustard and oil to a cup. Let the ingredients sit for one minute.
Place the immersion blender into the cup and starting slowly pulsing the mixture. In a few seconds you will see the mayo begin to form at the bottom. You can move the immersion lightly up and down to finish. The whole process should take no more than one minute. Transfer mayonnaise into a container; cover tightly and store in refrigerator for up to one week.