Instructions

Fill a large saucepan with lightly salted water and bring to a boil. Cut
the peeled potatoes into halves or quarters, add to the boiling water, and
cook 15 minutes, or just until just tender. Drain.
Push the potatoes through a potato ricer or food mill into a large mortar
or bowl; add the bread crumbs, garlic, egg yolk, salt and pepper. With a
pestle or wooden spoon, pound until fairly smooth and well mixed.
Gradually add the vinegar and then the olive oil, pounding constantly,
until the sauce is thick. Add more vinegar and olive oil if needed.
Serve immediately, or refrigerate in an airtight container for up to 3
days.