Melt the butter in a pan, add the peas and cook gently, covered, for 5 minutes. Blend in the flour and gradually stir in the stock; season with salt and pepper and add the mint sprig. Bring to the boil, then simmer for 10 minutes or until the peas are tender. Reserve a few of the peas for garnish.
Rub the soup through a sieve or purée in a food processor or blender. Return the soup to t