This sauce was very popular in Medieval age and sold by the streets vendors that usually came from the countryside where they collected fresh herbs. There are variations of the below recipe where the addition of ginger, cinnamon, sage, cloves, or some breadcrumbs has modernized the recipe.
1 Large bunch of Parsley
2 Anchovy Fillets in oil
1 tablespoon of Capers in vinegar
1 Garlic Clove
Salt and Pepper
Hard boil the egg then chop together the other ingredients.
A fine mixture is desired. If available, a food processor works very well.
Place in a bowl, add salt and pepper and dilute, stirring constantly, with good olive oil until the sauce is creamy.