1. Add egg, mustard, vinegar, and salt into your food processor or blender. Blend for 10-15 seconds until smooth.
2. Take the lid off of your food processor or blender, and, while blending, very slowly add the oil in small drops. Once the mixture begins to thicken, you can add the oil a little more at a time.
3. Once all the oil has been added, turn the food processor or blender off and scrape the bottoms and side of the bowl and then process for another 10 seconds. Taste the mayonnaise and add additional seasoning if desired. I like to always add a squeeze of fresh lemon juice to help bring out a little tartness, but that is totally optional.