Za'atar is a Middle Eastern spice blend typically containing dried thyme, oregano, marjoram, sesame seeds, and tangy sumac. The proportions vary by region, creating different flavor profiles from herbal and nutty to tart and aromatic. When using za'atar in baking, start with small amounts as the blend can be quite potent. The sesame seeds provide richness while sumac adds acidity that brightens baked goods. Quality za'atar should smell fragrant and have visible herb flecks and sesame seeds throughout.