How to Make It
In a large, sturdy mortar (preferably marble with a wooden pestle), sprinkle the garlic with 1/2 teaspoon salt, and pound and grind it to a relatively smooth paste. Add the egg yolk, and stir vigorously with the pestle until thoroughly combined and a shade lighter in color, 1 to 2 minutes.
Begin adding the oil, drop by drop, to the side of the mortar, while stirring constantly with the pestle. Once the mixture is thickened (indicating an emulsion has formed), begin adding the oil in a very thin stream. After about 1/4 cup of the oil has been added, it should be very thick and difficult to stir. At this point, stir in 1/2 teaspoon of water to loosen it.
Resume adding the oil, now in a slightly thicker stream, while stirring briskly with the pestle. As the mixture becomes too thick again, splash in another 1/2 teaspoon of water. Continue stirring in the oil and up to an additional 2 teaspoons of water until all of the oil has been added, 10 to 15 minutes. Stir in the remaining 1/4 teaspoon salt. Serve with desired accoutrements.