In your food processor, add the egg, water, mustard and apple cider vinegar. Let sit 1-2 minutes. Turn on food processor for 30 seconds until everything is mixed.
SLOWLY pour in the oil in a steady stream while the food processor is still on. You want to make sure that you are slowly pouring it in because that is the key to make the mayo thick & fluffy! This takes about 1 minute.
Use salt & pepper to taste. Store your mayo in a mason jar for up to two weeks!
Make sure the egg you use is brought to room temperature. You can leave it out the night before (if you remember!) or like me, just take it out of the fridge 30-60 minutes before making the mayo.
I use my food processor to make this mayo. I've heard that a high speed blender also works (I haven't tried) or an emulsion blender as well.