Melt the butter in a saucepan.
Sweat the onion until soft, but not coloured. Add the fennel bulb and cook for two minutes.
Add the vegetable stock and bring to the boil. Simmer for 6-8 minutes or until the fennel is cooked.
Stir in the double cream and cook until the sauce has thickened slightly. Season to taste and stir in the dill.
Place the clams in the base of a bowl and pour over the soup to serve.