Harissa (חריסה) is the North African chili paste that Tunisian and Moroccan kitchens carried into Israeli food, and as a sandwich element it is pure seasoning rather than filling. The angle is concentration. Harissa is dried red chilies softened and pounded with garlic, salt, and a spice base of caraway, coriander, and cumin, bound with oil into a deep red paste that is hot, smoky, and faintly bitter all at once. A sandwich that uses it is shaped around it: a thin smear carries the whole bite, so the build hinges on the rest of the components having enough body and fat to stand up to the heat without being flattened by it.
The way it works in bread is a matter of restraint and contrast. A film of harissa goes against the crumb, often loosened with a little olive oil or lemon so it spreads thin and threads through the filling instead of sitting as a hot lump. Its natural home in the Tunisian Israeli kitchen is the fricassee roll and the tuna sandwich: a small fried or crusty bread packed with oil-cured tuna, boiled egg, olives, capers, and preserved lemon, with harissa run through to tie the oily fish and the salt together and give the whole thing its edge. Done right, the paste is loose enough to coat without clumping, the heat builds rather than ambushes, and the caraway and garlic stay legible under the chili. Done wrong, it is pasty and raw-tasting, over-applied so the sandwich reads as only burn, or so timid it adds color and nothing else. The bread has to be sturdy, since harissa with oil and tuna will turn a soft roll to paste.
It travels far beyond the tuna roll. The same paste goes into shawarma and merguez wraps, over grilled fish and roasted vegetables, into a hummus plate for those who want the kick, and onto a plain cheese sandwich to wake it up. It varies mostly by region and intensity. The Tunisian style leans hot and caraway-forward and is often the reference; Moroccan versions can run milder and sweeter with more cumin and sometimes a thread of preserved lemon worked in; some cooks add roasted pepper for body, others keep it lean and fierce. Each of those preparations is recognizable on its own and deserves its own treatment rather than a line here, but they all return to the same idea: a single chili paste loud enough that the sandwich is designed around it rather than simply finished with it.