In the container of the device, place the lemon juice or vinegar, whole egg, mustard, and salt. Cover and blend for a few seconds. Adjust the speed at low rpm. While the device is running, add the oil through the opening in the lid, in a continuous stream (the mayonnaise will set quickly, in less than a minute). If necessary, stop the machine and use a spatula to clean the side of the container or to push the ingredients into the blades.