Add egg, mustard, vinegar, salt, and lemon juice to the dish you'll be blending in - food processor, blender, hand-mixer cup, or mixing bowl
Blend for about 30 seconds
Add a few drops of oil and continue to blend
Add a few more drops to get a good emulsion
Very slowly pour in oil in a thin stream while blending. It should take several minutes to do this. Mixture will thicken to a mayo texture during the process
Store in refrigerator