Melt 2 tablespoons butter in the top of a double saucepan and blend in the flour thoroughly, being very careful not to let it colour. Remove saucepan from heat and pour in the boiling stock. Cook over water, stirring constantly, until it thickens slightly; add finely chopped mushrooms and simmer for 10 to 15 minutes, stirring from time to time. Strain sauce; reheat and add cream and lemon juice. Season to taste with salt and a little cayenne. Remove sauce from the heat and whisk in the remaining butter, adding it in small pieces. Excellent for eggs, chicken and vegetables.
If the sauce is not to be used immediately, put several dabs of butter on top to prevent a skin forming.