Jolpai, the small, hard, sour Indian olive, appears in Bengal and Bangladesh markets from October to December, and no winter afternoon is complete without a jar of jolpai achar warming in the sun. It is no relation to the Mediterranean olive: it is tart and astringent, almost like a green plum. The olives are boiled, lightly crushed so the flesh splits from the stone, then cooked in mustard oil with panch phoron, dried chilli and plenty of jaggery. The pickle is sticky, sweet-sour and slightly tannic, with soft flesh to nibble off the stones. Eat it with khichuri on a cold evening, with dal-bhaat, or straight from the jar as a snack. The common mistake is under-boiling: the olives must be soft enough to press flat, or the syrup cannot get in and they stay bitter. It is done when the syrup is thick and shiny and clings to the olives. Very sweet and acidic, it keeps a month in a cool cupboard in winter and 3 months in the fridge. Abroad, look for frozen jolpai in Bangladeshi grocers; firm, sour green plums are the closest substitute. Chef's tip: keep a few olives whole and only scored; they give a nicer bite.