How to Make It
In a small saucepan of boiling salted water, cook the potato until tender, about 15 minutes; drain, cut them into 1/2-inch cubes. In a bowl, cover the bread with water and let soak until completely saturated, about 2 minutes. Transfer the bread to a plate.
Add the potato and process the mixture until smooth. With the machine on, add the oil in a steady stream until incorporated. Transfer to a bowl and refrigerate until chilled, about 1 hour. Season with salt and pepper. Garnish with parsley and serve.