Sumac is a tangy, brick-crimson spice ground from the dried fruits of the Mediterranean sumac shrub, Rhus coriaria. Bright, fruity-sour and gently astringent, it was the Levant's everyday souring agent long before lemons arrived — dusted over grilled meats, hummus and rice, stirred into fattoush, and bound with thyme and sesame into the za'atar blend.