Place the potato in a small saucepan and cover with lightly salted water by 1/2-inch. Bring to a boil and cook until tender. Drain and let cool.
Soak the bread well in water. Squeeze out the water and tear the bread into small pieces to form large wet bread crumbs. Set aside.
Place the garlic in a bowl and mix with the salt to form a paste. Add the potatoes, vinegar, and
lemon juice, and mash well. Add the wet bread crumbs and mash to make a smooth paste. Gradually add the oil, mixing constantly, to form a thick sauce. Continue to beat until the sauce is thick enough to hold its shape. Adjust seasoning, to taste, with salt and lemon juice. Spoon into a decorative bowl and serve at room temperature with Fried Calamari.