Put the oil in a wide, deep pan on a medium heat. Once it’s hot, add the cinnamon stick, curry leaves and mustard seeds, let them sizzle for 20-30 seconds, then add the garlic, ginger, chillies, lemongrass and turmeric, and cook, stirring, for two to three minutes, until fragrant. Add the leeks and half a teaspoon of salt, and cook for six to eight minutes more, until softened.