Crack the egg into a blender or food
processor and add the mustard and salt, to taste. Process for 30 seconds, then add the
wine vinegar and blend again. With the motor still running, pour in
the oil in a thin steady stream. The sauce will thicken after half of
the oil has been added. Continue to add the oil until it has
all been absorbed. Scrape the mayonnaise into a bowl with
a rubber spatula and store, covered, in the refrigerator.