Melt the butter in a large saucepan. Add the leek/onion and cook for 10 minutes, stirring from time to time, until very soft. Add the fennel seeds and cook for a further 2-3 minutes. Stir in the fennel and potatoes and cover with a lid. Cook over a low heat for a further 10 minutes until the vegetables are very soft.
Remove the lid and add the stock. Bring to the boil, then lower the heat and simmer, covered, for 20 minutes. Stir in the broad beans and simmer for a further 10 minutes.
Puree the soup in batches in a food processor or blender. Return the soup to the rinsed out pan and stir in the cream and add salt and pepper to taste.
Reheat gently but do not allow to boil.
Make the dill cream: Whip the cream to soft peaks. Stir in the dill and season to taste with salt and pepper. Ladle the soup into 4 warmed soup plates and top each with a spoonful of the dill cream. Garnish with the sprigs of dill.