This is the classic sauce, to accompany grilled meats and fish. It’s not a British sauce as such, but you can bet your life if you don’t serve it the customer will ask for it.
Put the vinegar, 2tbsp water, the shallot, herb stalks and peppercorns in a saucepan and simmer for a minute or until the liquid has reduced to 2tsp. Strain through a sieve and leave to cool.