1. Melt 3 tablespoons of butter in a saucepan.
2. Add a small chopped onion and the button mushrooms cut into very small pieces.
3. Season with salt and pepper, sprinkle with a tablespoon of chopped parsley.
4. Stir the mushrooms well in the hot butter. Cover and let cook over low heat for 15 minutes.
5. Then add 2 tablespoons of flour while stirring constantly. Add 1/4 liter of broth (or water) and 1/2 liter of milk.
6. Cook over low heat, stirring occasionally.
7. When the soup starts to boil, reduce the heat and let cook uncovered for another 15 minutes.
8. A few minutes before serving, add the juice of one lemon and then 2 tablespoons of fresh cream.