Pita bread, celery, carrot, bell pepper, and fennel sticks for dipping
Cover potatoes with water in a 3-quart saucepan and bring to a boil over high heat. Reduce heat to low and simmer until potatoes are tender, 15 to 20 minutes. Drain, rinse with cold water and process through a ricer or mash thoroughly.
Meanwhile, put garlic and salt in a large wooden bowl and pound with a pestle until thoroughly mashed. Gradually add potatoes and pound them with garlic. If mixture is still hot, let cool 15 minutes, then add egg yolk, if using, and vigorously mash or beat in with a wooden spoon. Alternating olive oil with lemon juice and vinegar, gradually add both to potato mixture, vigorously mashing or beating. Using a fork, add pepper, mixing briskly until very smooth. Mix in more olive oil, or even some water, a little at a time, if sauce is too thick to use as a dip.
Spoon into a container and store, covered, in refrigerator. It will keep for about 1 week. Bring to room temperature several hours before serving. Spoon into a bowl and serve with fresh vegetables and pita for dipping.
Adapted from Garlic, Garlic, Garlic: More than 200 Exceptional Recipes for the World's Most Indispensable Ingredient, by Linda and Fred Griffith.