Wash the oyster mushrooms well. Cut off the tough stumps and cut them into strips. Saute the chopped mushrooms in heated butter for 10-15 min.
Once the mushrooms soften, add the finely diced potatoes, finely chopped onions and saute another 15 min. on low heat. Sprinkle the ingredients with paprika, flour and braise briefly. Pour 4 cups of hot water and the salt over them, stir well and bring to a boil.
Boil the soup 20 min. on low heat, add the vermicelli and boil another 10 min. Take it off the heat, sprinkle with finely chopped parsley and black pepper. Thicken with yoghurt and serve.