Beat egg in a blender on low speed. Add lemon juice and seasoning, if desired. Continue to blend on low, or by pulsing it, and slowly drizzle in the oils through the small opening in the top. [if you don't have a blender, it works just as well with an immersion blender]
Continue blending until smooth.
It will keep for about 2 weeks in an airtight container in the refrigerator.