The Filipino sisig sandwich is built around a filling that was already a finished dish before it ever met bread, and that is the whole problem the sandwich solves. Sisig is chopped pork, face and belly and ear, boiled then grilled then crisped on a hot plate and dressed with calamansi, chili, and onion, served sizzling and meant to be eaten the moment it stops spitting. Putting it in a bread is a structural argument with a dish designed to be loose, fatty, and aggressively hot off the griddle. The sandwich works only if the carrier can absorb that fat and acid without going to slick paste, which is why the bread choice does almost all the work here.
The craft is in matching the carrier to a filling that fights back. Pandesal, the soft, faintly sweet Filipino roll, has a tight, slightly dry crumb that soaks rendered pork fat and calamansi juice without dissolving, and its mild sweetness is a deliberate counter to the sour-salty-spiced filling rather than competition with it. A brioche bun does the same job from the richer end, trading the dry-crumb absorption for an enriched softness that yields to the meat. The chopped texture matters as much as the bread: sisig is cut fine and crisped at the edges, so it packs into a roll evenly instead of sliding out in slabs, and the crisp bits give the only real bite in an otherwise soft sandwich. Calamansi is the load-bearing acid, sharper and more floral than lime, and it is what keeps a fatty pork filling from reading as one heavy note. An egg, often folded through the hot sisig until it just sets, binds the chop into a single mass the bread can actually hold rather than a pile that falls through the seam.
The sisig sandwich belongs to the dense long tail of regional American specialties, the place-and-community sandwiches built around a technique that only makes sense where it comes from, in this case the Filipino-American kitchens of the Bay Area and Los Angeles. Its relatives in that catalog share only that they belong somewhere specific, and each deserves its own article rather than being crowded in here.