Peel and quarter the potato. Cook in salted water until tender but not mushy.
While the potato cooks, crush the garlic and salt using a mortar and pestle.
Drain the potato, reserving some liquid to thin the dip later if necessary. Let the potato sit for 5 minutes to dry a bit.
Mash the potato. Add garlic paste, lemon, and vinegar and mash some more to mix.
Whisk in the olive oil in 3 or 4 additions. Taste and adjust seasoning. Garnish if desired. Serve at room temperature. at room temperature.