In a blender or food processor, blend egg, lemon juice, ACV and mustard powder.
Combine oils and slowly* add into the food processor, one tablespoon at a time, continuing to blend.
Add the salt once a creamy mayo has formed.
Transfer into a jar and store in the fridge for up to 2 weeks.
*And we mean slowly. The secret to this mayo is patience. Otherwise, it will split and you will be left with liquid, rather than a thick creamy blend.