Start by collecting all your ingredients. Make sure your egg is at room temperature, as this helps in emulsifying the oil and creates a smoother mayonnaise.
In a tall, narrow container (or a jar if using an immersion blender), add the egg, distilled white vinegar, lemon juice, Dijon mustard, and salt.
Using your immersion blender, blend the ingredients together for about 10 seconds until fully combined.
With the blender running, slowly drizzle the vegetable oil into the mixture. It’s essential to do this gradually to allow the emulsion to form. You’ll notice the mixture thickening as you add the oil.
Once all the oil is incorporated and the mayonnaise is thick, taste and adjust the seasoning. Add more salt or a splash more lemon juice, if desired, to balance the flavors.
Pour your homemade mayonnaise into a clean jar with a lid. It can be stored in the refrigerator for up to a week.