Sauté onions in butter until tender. Add flour and cook for 2 minutes without browning. Add stock and seasonings. Remove the stems from the mushrooms and chop; add to stock and simmer, partially covered, for 20 minutes. Strain, pressing juices from throw-away stems. Return soup to pan. Melt the 2 tablespoons butter in a skillet and sauté the mushroom caps, which have been sliced. Add seasoned salt and lemon juice. Add mushrooms to soup base. Simmer 10 minutes. Beat egg yolks with whisk into cream and slowly stir into soup. Add brandy; heat, but do not boil. Top with chopped parsley to serve. Serves 8 to 10.