Beat egg yolk in a large bowl for 1 minute on low speed using a blender or hand-held egg beater until it is thick.
Add the mustard, vinegar, and salt and beat for another 30 seconds.
Slowly drizzle oil into the egg mixture with the blender/beater on high. This will take several minutes. Do not stop blending or beating until the mixture thickens.
Taste your mayonnaise and season with extra salt, pepper and vinegar as desired.
Store in a glass jar in the refrigerator for up to 3 weeks.
Makes approximately ¾ cup, easily doubled to make more