This soup is great as is, or with a few shreds of cooked chicken added to each soup bowl. The sumac garnish looks lovely and it’s a perfect compliment with its bright, citrusy flavor. Avgolemono is even better after it has had time to sit and gain body, creaminess, from the rice. Make it early in the day, or a day in advance, and reheat it carefully over medium low heat.
In a medium saucepan, heat the stock to boiling and reduce to a simmer.
In a blender, puree 1 cup of the hot stock, ½ cup of the cooked rice, and lemon juice. Add the egg yolks and puree again.
Add the remaining cooked rice to the simmering stock, then add the egg mixture about ½ cup at a time, stirring to combine after each addition. Taste and season with salt and pepper. Simmer until the mixture is slightly thickened.
Serve the soup in warmed bowls with a pinch of sumac on top of each one.