Tahini (طحينة) is sesame paste turned into a sauce, thinned with lemon, water, and garlic, and in a sandwich it functions as structure rather than seasoning: the moisture, the fat, and the binding that holds dry fillings together in bread. The angle is that raw tahini and tahini sauce are two different things, and the sandwich depends entirely on the second. Straight from the jar, tahini is thick, bitter, and pasty, useless as a sandwich element. Loosened with acid and water it becomes a pourable, nutty, lemon-sharp cream that coats every surface it touches. That transformation is the whole job. Done right, the sauce makes a handful of dry fritters or boiled chickpeas into a coherent sandwich; done wrong, too thick and bitter or too thin and bland, it either clumps or runs off and the filling stays dry.
The role is consistent across the sandwiches it appears in. Tahini is whisked with lemon juice, crushed garlic, salt, and water until it loosens, sometimes brightened with parsley, then used as the binding coat. In a falafel sandwich it is what moistens the crisp shell and the dry herb crumb. In a foul or hummus roll it is the layer that ties the legumes to the bread. With grilled fish, shawarma, or roasted vegetables it is the rich, sour sauce that fills the gaps and carries flavor into every bite. The discipline is consistency and placement. The sauce should be loose enough to flow but thick enough to cling, and it has to touch both the bread and the filling rather than pooling in one place, so it is usually streaked on the bread first and drizzled over the filling second. Whisked too thick it sits in pasty lumps and the bitterness shows; thinned too far it slides to the seam and waterlogs the bread; under-acidified it tastes flat and heavy against the food it is supposed to lift.
It varies mostly by how thin it runs and how hard the lemon and garlic hit. Some kitchens keep it pale and mild, a quiet richness behind the filling; others push the acid and garlic close to a sharp, almost toum-adjacent edge that becomes a flavor in its own right. The sesame itself matters, with fresher, less bitter paste giving a cleaner nutty base and stale paste giving a muddy, heavy result. Where a sandwich uses a different binder entirely, garlic sauce, yogurt, or hummus in tahini's structural place, those belong to their own treatments rather than a footnote here. What stays constant is the function: sesame paste made into a sauce that gives a dry filling moisture, fat, and a sour-nutty lift, judged on whether it coated cleanly and held the sandwich together.