(Hand method)
Combine garlic with mashed potatoes or bread in a mortar.
Blend to form a soft paste. Add olive oil and vinegar
alternately in very small amounts, stirring briskly. Add
salt and pepper. Continue to whisk until sauce is thick.
(Blender or food processor method)
Place garlic and potatoes or bread in bowl and blend. Add
olive oil and vinegar and continue to blend. If too thick,
add 1/4 cup cold water. Add salt and pepper. Blend until
creamy and thick.