In a medium bowl, whisk together the lemon juice, egg yolk, mustard and salt. Add the first third of the oil drop-by-drop, whisking constantly. When the emulsion begins to form, add the remaining oil in a thin, steady stream, whisking constantly. Season with pepper.
Food processor or blender method
Place the vinegar, whole egg, mustard and salt in a food processor or blender. Cover and mix for a few seconds. Set the appliance to the lowest speed. With the machine running, add the oil in a thin, steady stream through the opening in the lid. The mayonnaise will emulsify quickly—in less than a minute. If necessary, turn off the processor or blender and scrape the sides of the bowl so the blades can reach the ingredients.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.