Choose Your Method: You can make mayo using a blender, food processor, or an immersion blender. The immersion blender method is the easiest and most foolproof.
Combine Ingredients: In a tall jar or a food processor, add the egg, lemon juice (or vinegar), mustard, and salt.
Slowly Add Oil: If using a food processor or blender, start blending and very slowly drizzle in the oil. If using an immersion blender, pour all ingredients in the jar and place the blender at the bottom before turning it on.
Blend Until Thick: Blend until the mixture thickens and turns into creamy mayonnaise. This usually takes about 30 seconds with an immersion blender and a minute or two with other methods.
Taste and Adjust: Add more salt or lemon juice if needed.
Store: Transfer to an airtight container and refrigerate. Homemade mayo lasts about one week in the fridge.