In a blender, combine egg, lemon juice, and mustard powder and blend until combined
While the blender is running on a medium to low speed, slowly pour the avocado oil into the blender. Pouring all the oil into the blender should take a couple of minutes. That's how slow you should pour!
Once all the oil is in the blender, leave the blender to run for about 20 seconds to make sure all the oil has been emulsified with the egg mixture
Use immediately or store in a glass container in the refrigerator until serving