Using your food processor or blender, combine the egg, salt, lemon, and dry mustard. If you would like to add anything else, you would add it now and pulse until combined. (If you prefer a chunky consistency you can add your additions after you have processed the mayo).
Turn the processor on high and slowly drip the oil in, the slower the better! Do not rush this step. Take it from me, if you pour the oil in too quickly you will have a liquid mess.
You should see the mayo start to become fluffy and thick as you slowly drip the oil in. As soon as all the oil is added, transfer to a glass container and you can store in the refrigerator for up to a week!