Place the egg, 2 tbsp. of vinegar, dried mustard and salt into a blender. Blend on high speed for about 10 secondsTurn the speed to low and very slowly drizzle in the olive oil. Make sure the stream is really thin and you pour very slowly. This helps to emulsify everything smoothly. Taste for seasoning and add more vinegar or salt if you'd likeStore covered in the fridge for a week