In a deep pan in fat and less water sauté the chopped onion together with rice. Add 4-5 cups boiling water. After a few minutes add the cleaned and sliced ​​mushrooms, chopped onion and chopped rhubarb. Broth leave on low heat for ten minutes. Then add the chopped Tharos and sorrel. Comminute the feta cheese and add to the broth. Salt. Stir the egg. Mix the yogurt and mix well again. While stirring the soup thicken, so as not to cross.