The focaccia sandwich is the rare one where the bread is the lead rather than the carrier. Focaccia is an Italian oil-enriched bread with an open, irregular crumb, a chewy interior, and a surface that has had olive oil pressed into dimples across it before baking, so it goes into a sandwich already seasoned and already rich. Most sandwich breads exist to stay out of the way of the filling. Focaccia refuses that role: its oil, its salt, and its loose crumb are loud enough to be tasted in every bite, and the build has to be designed around a bread that is a flavour in its own right rather than a neutral surface for one.
The craft is working with that oil and that crumb instead of against them. The slab is split horizontally and the open crumb soaks dressing and the juices of the filling readily, which is an asset when the fillings are robust and a liability when they are delicate, so this is bread for assertive loads: cured meats, roasted vegetables, a strong cheese, things that can stand next to an olive-oil bread without being erased by it. Light toasting or pressing firms the cut faces so the structure holds and the surface oil crisps slightly rather than sitting slack. The filling is kept relatively dry because the crumb is already carrying oil and will turn greasy and collapse if a wet dressing is added on top of it. A herb such as rosemary on the crust is part of the seasoning, not a garnish, and it is allowed to read.
The variations stay close to the Italian deli counter the bread comes from. Cured ham or salami with a hard cheese leans on salt against the oil; roasted peppers and aubergine with mozzarella keeps it meat-free and still robust; a pesto or tapenade smear treats the bread as the partner rather than the wrapper. The wider regional-bread shelf, the ciabatta with its own open crumb, the sturdy bloomer, the sourdough, makes the same argument that the loaf can be the point. Each deserves its own article rather than being crowded in here.