While the coals of the barbecue are burning hot, char the peppers, turning frequently using metal tongs until blistered and evenly blackened (see tip below for charring indoors). Remove to a bowl, cover with a plate and set aside to cool completely – this will create steam, which softens the flesh and makes the skin easier to remove.

Gently rub the peels off the peppers and remove the seeds. Finely chop the flesh on a board, then scrape back into the bowl along with any juices. Mix in the pesto or basil, olive oil and vinegar, and season well. Can be made up to two days ahead and chilled until needed. Remove from the fridge 1 hr before serving. Spoon over the fish and veg.