Add the onion to the casserole and sauté for 5 minutes, until softened. Add the garlic, chilli, curry paste, lime leaves, star anise and cinnamon stick. Cook for 30 seconds, then pour in the coconut milk, beef stock, fish sauce and treacle, if using, and bring to the boil. Add the beef pieces back to the casserole, cover with a lid and reduce the heat. Simmer for 1¾ hours, until the meat is meltingly tender and the liquid has reduced. Simmer for an extra 30 minutes without the lid on if needed.