Scrape the mussels carefully. Wash them in several waters without letting them soak. Peel, then chop the onions, chop the parsley. Heat the butter in a large skillet, add the onions, let them melt without taking color. Pour in the beer, mix well, add the bay leaf and then the mussels. Season with pepper. Cover, cook over high heat, shaking the pan. As soon as the mussels are open, stop the cooking. Add the cream and mix well. Pour into a serving dish, sprinkle with parsley and serve immediately. Serve with fries.