#Using nigoise olives will give the finished sauce a truly authentic flavour. It is preferable to buy whole olives and stone them yourself - to do this, simply press down firmly on the olives using your thumb, and the olive flesh will split to reveal the stone, which is then discarded.#
Put the olives, anchovies, garlic, capers and mustard in a mortar (or food processor) and pound with a pestle to form a fairly smooth paste.
Gradually blend in the oil and add lemon juice and pepper to taste. Transfer to a dish, cover and refrigerate for up to 5 days.