Add egg to the small bowl of a food processor and process for 20 seconds.

Add the mustard, vinegar, and salt. Process for another 20 seconds.

Scrape the sides and bottom of the bowl, turn the food processor on then begin to slowly add the oil in tiny drops until about a quarter of the oil has been added (this is critical for proper emulsification).

When you notice that the mixture is beginning to thicken and emulsify, you can be a little less strict. With the processor on, continue to add it slowly, but increase to a very thin stream instead of drops of oil.

When all of the oil has been added, scrape the bottom and sides of the bowl and process for an extra 10 seconds. Taste mayonnaise for seasoning then add salt, lemon juice or extra vinegar to taste.

If the mayo seems too thin, slowly stream in more oil with the processor running until thick. If it is too thick, add a teaspoon or two of cold water.

Add all of your mayonnaise ingredients (including the oil) to a tall jar, then turn on the immersion blender, slowly moving it up and around to incorporate the ingredients and blend into a thick mayo.

Remove the blender, taste, and then add additional salt or lemon juice to taste.