Sumac (سماق) is the tangy red spice that functions less as a topping and more as the acid line in a sandwich, ground from dried sumac berries and used to add sourness where lemon would add liquid. The angle is that sumac is a dry souring agent, and that property is what makes it structural in bread rather than incidental. A wet acid like lemon juice or vinegar brightens but also softens, and in a rolled or pressed sandwich that moisture can work against the bread. Sumac delivers a clean, fruity tartness with no water at all, so it can sit directly against onion, grilled meat, or labneh and sharpen the whole thing without making anything soggy. Used well it cuts richness and lifts flavor; used badly it either disappears under stronger seasoning or turns the bite dusty and astringent.
In a sandwich the role is consistent across the dishes it appears in. Sumac is most associated with onion: thin-sliced raw onion tossed with sumac and a little salt becomes the sharp, sour, almost pickled layer that defines shawarma and many grilled-meat builds, where it cuts the fat of the protein the way a pickle would but with a different, fruitier acidity. It is also dusted directly over labneh, fattoush in a roll, grilled chicken, or fried eggs as a finishing line, and it is a core component of za'atar, so it reaches the manoushe through that blend as well. The discipline is dosage and placement. A scatter against onion or over a spread reads bright and clean and ties the richer elements together. Too heavy a hand turns the surface gritty and the flavor harshly sour, and seasoned too early it can go flat by the time the sandwich is eaten, so it usually goes on close to assembly.
It varies mostly by how it is deployed and what it is paired with. Mixed into an onion garnish it is sharp and present and reads almost like a condiment in its own right; dusted lightly over labneh or eggs it is a quiet acid note that rounds the dairy or fat without announcing itself; folded into za'atar it is the sour backbone behind the herb and sesame. The quality of the berry matters too, with brighter, less salty sumac giving a cleaner fruit-acid and dull or oversalted product giving a muddier, flatter result. Where lemon or vinegar would do the same job with added liquid, those wetter approaches belong to their own treatments rather than a footnote here. What stays constant is the function: a dry, fruity acid that sharpens and lifts a sandwich without adding moisture, judged on whether it cut the richness cleanly and stayed bright.