Wash the parsley and basil in cold water. Spin dry and roll up between paper towels to absorb any extra water. Alternative, dry between multiple layers of paper towels or a clean tea towel.
Pick basil and parsley leaves, discarding the large stems (small stems are fine) and refrigerate until required.
Peel garlic cloves and crush them into the bowl of the food processor.
Press anchovy fillets between paper towels lightly to remove excess oil.
Add the capers, anchovies, mustard, and 1 tablespoon of vinegar and 1 tablespoon of olive oil and pulse until chopped finely. Scrape down the bowl several times to ensure even chopping.
Add parsley, pulse a couple of times and scrape down the bowl. Then add the basil and pulse until herbs are finely chopped (not too fine you don’t want a puree).
If using for a marinade remove two tablespoons of the herb mix for the marinade at this stage.
Whisk in remaining vinegar and olive oil (you may not need it all – taste as you go – I like a good bit of vinegar myself).
Add salt and pepper to taste.
Baste the roast chicken with the sauce as soon as it comes out of the oven. Serve the remaining sauce with the chicken on the side.