1 egg yolk
1/2 c yogurt
This is a way to thicken soup. Whisk the egg yolk with the yogurt in a bowl. Gradually add a ladle or two of the broth to the zastroika, stirring well to temper the temperature. Pour the zastroika into the soup, stirring. If you need to warm the soup, do it on a gentle heat to prevent scorch and curdle.