Heat the groundnut oil in a large, heavy-based saucepan over a moderate heat. Sweat the shallots, garlic and ginger for 3-4 minutes, stirring occasionally, until they start to soften. Add the chilli, lemongrass and star anise and cook for a further minute. Pour the stock in and bring to a simmer. Add the chicken and simmer for 6-8 minutes until cooked through, with no pink showing. Meanwhile, whisk together the lime juice, sugar, fish sauce and light soy sauce until combined. Remove the soup from the heat and stir this mixture gradually into the soup to season. Also season with black pepper, then stir in the beansprouts and coriander leaves. Ladle into soup bowls and serve immediately. Freezing and defrosting guidelines Make the soup, then leave to cool at room temperature. Freeze (without garnishes or toppings) in a rigid container, leaving a bit of space for expansion, for up to 1-3 months. Reheat either from frozen or defrost in the fridge overnight. Once piping hot, add toppings or garnishes and serve.