In a large saucepan bring salted water to a boil. Add the potatoes and cook 15 minutes, or until tender. Drain and cool. Peel the potatoes. Push them through a ricer into a bowl. Pound the garlic with salt in a mortar and pestle until smooth and add to the potatoes. Gradually add the vinegar, lemon juice, and three-fourths cup of the oil, pounding or stirring constantly. Taste and season with salt and freshly ground pepper to taste and add more oil if desired.
Makes about 4 cups.