Heat butter in a saucepan. Add finely chopped onions and saute it lightly. Add sliced mushrooms. Cook on a low flame for 5 to 6 minutes. Add dried thyme and refined flour. Mix well and cook for 2 to 3 minutes. Add the mushroom stock. Stir and add salt black pepper powder, and heavy cream. Bring the soup to a boil. Simmer on a low flame for 4 to 5 minutes. Garnish the soup with Parmesan cheese and a parsley sprig. Serve hot.