A 20-minute stint in a pan can work wonders on ordinary white onions, turning them into a wondrous, meltingly sweet base for so many things – not least in this moreish gratin. If you use jarred chickpeas, with their extraordinarily tender texture, this is a one-pan dish, and you need do no more than tip them in towards the end, before flashing the dish under the grill. Tinned chickpeas need a little more help (I outline both methods below). This is one for a cold night, with a big glass of red wine.
