Start with a clean bowl and ensure all ingredients are at room temperature.
Add the egg, Dijon mustard, and vinegar to the bowl.
Use an immersion blender to mix the ingredients on low speed.
Slowly drizzle in the oil while blending continuously.
As the mixture begins to emulsify, you can increase the speed.
Continue adding oil until fully incorporated and thickened.
Season with salt and lemon juice to taste.
Transfer the mayonnaise to a clean jar.
Store in the refrigerator for up to one week.
Enjoy your homemade mayonnaise in various dishes!