Stir the flour and salt into the yogurt, then add the water and beat well. In a stockpot, sauté the onion lightly in the butter, then stir in the yogurt. Bring this mixture to a boil and simmer for 15 minutes. Carefully stir a little of the hot liquid into the beaten eggs, then whisk the eggs into the soup. Simmer a few minutes longer. Just before serving, add the minced herbs and rice.