
Cut the crusts from the slice of stale bread and cut the bread into small pieces. Add to a small bowl and cover with white wine vinegar, set aside.
Put the egg in a small saucepan and fill it with cold water. Cover with a lid and bring to a boil. As soon as the water is boiling, remove the lid and put a timer on for 7 minutes.
After 7 minutes, drain the egg into a sieve and run it under cold running water for about 30 seconds. Set aside to cool, then peel the egg from its shell.
Put the parsley (with stalks) in a food processor with the anchovies, capers and garlic. Remove the yolk from the hard-boiled egg and add it to the food processor.
Squeeze the bread to remove the excess vinegar and add it to the food processor, followed by the olive oil. Blend to a smooth sauce, adding more olive oil if needed.
Taste for seasoning and add salt and pepper as needed.