Melt the butter in a pot and add the onion and mushrooms. Cook over low heat for 15 minutes, stirring occasionally. Sprinkle with the flour and cook for a few minutes more. Slowly add the stock, and heat, stirring, until it reaches the boiling point. Reduce the heat and simmer for 20 minutes. Stir in the cream and season to taste. Reheat before serving. COLD MUSHROOM SOUP: Use an additional quarter cup of heavy cream and chill. Adjust the salt and serve in cold soup bowls with chives sprinkled on top. ----------------------------------------------------------------------------