You don't need to know exactly what goes into za'atar, a Middle Eastern spice blend. It's one of those things that's shrouded in secrecy, with proprietary blends and family secrets obscuring its exact composition. It typically involves dried thyme, oregano or marjoram mixed with sesame seeds, salt and sometimes sumac, or cumin, or coriander or ... you get the idea.
The beautiful thing about za'atar is that, just like it can contain all sorts of ingredients, it tastes great with all sorts of ingredients, too. Use it as seasoning for sautéed vegetables or grilled chicken. Mix with olive oil and eat with toasted bread. Sprinkle it on olives, hummus or salad. And since you have that nice yogurt on hand already, go ahead and shower it with za'atar as well, just as they've been doing in the Middle East for generations.