Stir sugar, vinegar, onions, ginger, garlic, mustard seeds, coriander, crushed chillies and salt into the pan, then drop the cinnamon stick on top. Heat slowly for about 20 mins, giving an occasional stir, until the sugar has dissolved. Now leave the chutney to simmer at a steady pace, without a lid, stirring occasionally until reduced and thickened, but still with a nice balance of syrupy juice. This will take about 1½ hrs. You don’t want to cook it until all the liquid has gone, as it will thicken once it cools. Discard the cinnamon stick.