Crack the egg into a wide mouth mason jar or other cylindrical container. Add the remaining ingredients.
Place the immersion blender all the way down into the bottom of the jar. Turn on and blend for about 20 seconds. Without stopping the blender, slowly begin to lift it up from the bottom of the jar, circling slowly to ensure that all of the oil gets blended.
Once the mixture has a creamy, spreadable consistency, place a lid onto mason jar or transfer mayonnaise to another container. Store in the refrigerator for up to a week.