In a wide-mouth pint Mason jar (or other jar that will fit the blade of an immersion blender), add in order the egg, garlic, dijon, salt, lemon zest (if using), lemon juice and last the oil
With the immersion blender in the OFF mode, place the blade of an immersion blender at the bottom of the jar, holding it straight up and down
Turn the immersion blender on low, keeping the blade at the bottom of the jar until the mayonnaise starts to come together, about 30 seconds
When the mayonnaise begins to form, it will get thick and turn white, slowly raise the immersion blender up to the top of the oil line
Once you have reached the top move the blade up and down a few more times until the remaining oil has been incorporated