Add the lardons and onion to a very large saucepan placed over a medium heat. Stir occasionally to prevent burning until the onions are soft and no liquid remains; add the mushrooms, stock and a little seasoning. Turn up the heat to bring to a boil, cover with a lid and reduce to a simmer for about 10 minutes or until the mushrooms are fully cooked. Remove from the heat to cool before adding the cream and parsley. With a hand blender or liquidizer, blitz until smooth. Serve piping hot with a garnish of crème fraiche, grated Parmesan and chopped parsley.