Blend the egg, apple cider vinegar and dijon mustard together in a container that you can comfortably pour oil into while blending.
Slowly add the avo oil while blending. It is VERY IMPORTANT to add the oil very slowly in the beginning and to keep blending the entire time. After a short bit you will see and feel the mix starting to thicken. Once you are 100% certain that the consistency of your mix is thickening you can start pouring the oil in a bit faster (but still not too fast!).
Keep adding oil and blending until all the oil is used.