After allowing the egg to reach room temperature (leave out of refrigerator for 2 hours), carefully remove the egg yolk from the egg white. Dispose of egg white or refrigerate and save it for something else.
In the mason jar, add egg yolk, olive oil, apple cider vinegar, mustard, and salt.
Place immersion blender into the bottom of the jar, touching the bottom. Turn stick blender on and hold at the bottom of the container for 30 seconds. The mixture will quickly become the consistency of mayonnaise.
After 30 seconds of blending, turn off immersion blender. Bring immersion blender blade to top of mayonnaise mixture, turn on, and move the blender straight down. Do this three to four times, always turning the immersion blender off once the blade has reached the bottom of the container and turning it back on only once the blade is back to the top of the mixture.
Once emulsified, serve on your favorite savory keto dishes or refrigerate immediately.