A friend's brother taught me how to make this when we were in high school- over twenty years ago (ouch!). I doubt that he still remembers it.
It originally calls for labaneh, a middle-eastern cheese. You can make your own labaneh by mixing plain yogurt with a little salt, and draining it in a cheese cloth for a few hours. I've made it with sour cream, as well, and it's fine, although the consistency is different. —Hummusit