Add egg, mustard, lemon juice, salt, pepper, and oil to the jar.
Place the immersion blender in the container; it should sit on the bottom of the jar.
Turn it on, keep it fixed there and count to 10. The mayo should start to emulsify. Slowly, begin moving the immersion blender up and down, counting to 10-15 again; it should not take much longer than that. The mayonnaise will now be fully emulsified, thick, and glossy. If it is not yet, blend for another 3-5 seconds or so.
Remove the immersion blender from the jar and stir the mayo.
Flavor: Adjust the taste with more salt, if necessary, lemon juice (1-2 teaspoons extra, to taste), and add other ingredients, if desired (See blog post for suggestions).