Add the egg in gently to the mason jar. If the yolk breaks, you will more than likely have to use another egg.
Next, add in the mustard, salt, and lemon juice. Pour in the avocado oil on top.
Now, here's the tricky part. You will NEED an immersion blender. You cannot do this with a typical blender for it will not form properly. What you'll need to do is place the immersion blender at the bottom of the mason jar & turn it on. Hold it there until you feel pressure from the mayo on the bottom. It will more than likely take 30-60 seconds. The consistency should be thick.
If it's not thick like mayonnaise, I would add another egg to the bottom and do the whole process all over again.
Ready to serve and store in the fridge.