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.