Remove any coarse stalks from the parsley. Place the parsley, rocket or herbs on a chopping board and use a sharp knife to chop very finely. Put the anchovies on top and chop until well-blended, then repeat with the capers, if using. Transfer to a bowl and whisk in the oil and vinegar. Taste, adding a touch of salt or a little more vinegar, if desired. It should be a thick-ish but drizzle-able sauce.