Turn on the broiler and line a baking tray with foil. Arrange the peppers, skin-side up, on the baking tray and brush with oil. Broil for 10-15 mins, turning, until skins are well charred. Put the peppers in a plastic bag and seal for 5 mins to loosen the skins.
Meanwhile, crush the garlic and a generous pinch of salt to a paste using a pestle and mortar. Add the vinegar and oil, and blend to make a dressing. Strip the skins and seeds from the peppers, then quarter. Put the flesh in a bowl or food container. Pour over the dressing and scatter with the pinw nuts, capers and basil if using.