Heat the butter in a pan and sweat the onion and fennel very slowly without colouring until soft. Add the potato, fennel seeds and white wine and simmer for about 10 minutes. Add the stock and simmer for around 20-30 minutes until the vegetables are completely tender.
Liquidise in a blender, return to the pan and pour in the cream, if using. Bring back to a gentle simmer and season. Serve garnished with the reserved fennel fronds.
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.