Add ingredients to a jar. Place egg, ground mustard, lemon juice, salt and avocado oil into a wide mouth mason jar, in the order listed. Let the ingredients settle for a minute or two.

Immerse. Place the immersion blender on the egg, allowing the blender to sit on the bottom of the jar. Blend for 5 seconds or until the egg has emulsified and turns white and creamy. Slowly work your way up the jar, watching the ingredients solidify, for about 30 seconds. If you find pockets of oil, continue to run your immersion blender through the jar until all of the ingredients have come together into a thick but creamy mayo.

Enjoy or store. Serve mayo right away or store in the fridge, in an airtight container for up to two weeks.