Gather all your ingredients and tools. Ensure your egg is at room temperature for better emulsification.
In your mixing bowl, add the large egg, Dijon mustard (if using), and lemon juice. Use a whisk or your immersion blender to combine these ingredients until well mixed.
With your immersion blender running, gradually pour in the olive oil in a slow, steady stream to create the emulsion.
Continue blending until the mayonnaise thickens to your desired consistency. Adjust the amount of oil based on thickness preference.
Taste your homemade mayonnaise and adjust seasoning as necessary. Add more lemon juice for tang, salt for flavor, or more mustard for an extra kick.
Transfer your mayonnaise to an airtight container and store it in the refrigerator. It should last for about a week.
