Place the egg yolk in a bowl and
gradually beat in the salt, mustard and pepper to taste, using a
balloon whisk. Add the oil, drop by drop, whisking
vigorously after each addition so that is absorbed completely before
the next addition.
After about half the oil has been
added, the mayonnaise will start to look shiny and thick. If so, the oil can now be added more
rapidly, about a tbsp at a time. As the mixture becomes very thick, add
a little vinegar to soften it, then add the remaining oil.
Blend in the rest of the vinegar. Adjust the seasoning to taste. The consistency should be like whipped
cream. Use straight away, or store in the
refrigerator in a covered container for up to 2 weeks.