Slice mushrooms, removing the stems. Cook stems in 1 cup of water and save the stock to use later. Melt in saucepan one-half the butter, add mushrooms and squeeze the lemon juice on the mushrooms. (No seeds). Add 3 tablespoons mushroom stock and cook slowly until mushrooms are soft. In another pan heat the other half of the butter and add flour. Add wine to mushroom stock, bring to boiling point and add mushrooms. Heat again and remove from fire. Mix some of the liquid with egg yolks and add very slowly to sauce. Season. Heat again, do not boil. Strain through sieve, add mushrooms and serve immediately. This can be made with canned mushrooms. Save juice for stock.