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Put the olive oil in a large frying pan over a medium heat and fry the shallots until golden and caramelised, about 10 mins.
Add the port, wine and herbs and simmer for 10 mins, or until reduced by half.
Pour in the stock and continue to cook until reduced by half again, then strain, discarding the shallots and herbs.
Transfer the sauce into a new pan and bring to the boil. Remove from the heat and whisk in the butter, then season to taste.