Place garlic and salt in a large mortar and pestle, and pound to a paste. (Alternatively place garlic and salt on work surface, and mash with the side of a knife to form a paste and transfer to a stainless steel bowl.) Add egg yolk, and whisk to combine. Drizzle in half the oil, one drop at a time, whisking constantly. As the mixture begins to thicken, add 1 teaspoon vinegar, 1 teaspoon lemon juice, and 1 teaspoon water.
Pour in the remaining oil in a slow steady stream, scraping down sides of mortar as necessary. As the mixture thickens, add a little more lemon juice, vinegar, and water, and continue whisking until the remaining oil is completely incorporated and the sauce is thickened. Season with lemon juice and salt.