To make the sauce, heat the butter in a large pan, then tip in the onions, garlic, ginger, spices and bay. Fry over medium heat for 8-10 mins until the onions colour. Stir in the purée, curry powder and coriander stalks, then cook for a few mins until everything is sticky. Pour in the stock and cream, season, stir well, then bring to the boil. Gently simmer for 15 mins until reduced by half. Strain through a sieve into a container. Keep covered in the fridge for up to two days.