To make a quick mushroom sauce to serve with the pancakes, heat a little oil in a clean frying pan. Fry the chopped onion on a low heat for ten minutes until golden and soft. Add the mushrooms and cook for a further few minutes, until soft. Pour in the stock and simmer for one minute, then pour in the double cream and stir in the sour cream. Allow to bubble and simmer, then serve with the potato pancakes, along with some fresh dill and extra sour cream on the side.