In a jar, combine the egg, lemon juice, apple cider vinegar, mustard, and olive oil.
Push your stick blender right down to the bottom of the jar and whizz ingredients together without moving the stick blender until the mayo is at least 75% emulsified. Once it's about 75% emulsified you can slowly move the stick up and down to emulsify the rest. (Make sure you only blend for about 20-30 seconds, over blending will cause it to be thin and runny!)
Check the seasoning and add more salt or or lemon juice if needed.
Pop on a tight-fitting lid and refrigerate. Allow it to chill for at least an hour before using it, and use within 1-2 weeks.
