The Pain aux Olives Garni is a sandwich whose bread is already seasoned. Pain aux olives is a wheat loaf, often enriched with olive oil, with chopped black or green olives worked through the dough before baking, so the crumb arrives salted and savoury throughout rather than waiting for a filling to flavour it. A plain baguette is a blank base; this loaf is not. Split it and the bread is contributing olive salt and oil to every bite before anything is laid inside, which changes the job of the filling entirely.
Because the bread is doing the seasoning, the fillings have to stay light or they collide with it. The Provençal versions keep it spare: a few slices of jambon cru, a soft fresh cheese, a little roasted pepper or tomato, a leaf of basil, sometimes nothing more than good oil and a tomato rubbed across the cut face. A heavy, sauced, or strongly cured filling buries the olive note that is the only reason to use this bread instead of a plain one. The crumb is firm and a touch chewy, holds oil without going slack, and tears rather than snaps, which suits a filling that is itself oil-dressed rather than dry. This is southern eating: a sandwich that starts from flavour and asks the filling only to stay out of the way of the loaf. Provence is where the bread and this restraint belong, and the sandwich reads as a Provençal one before it is anything else.
The variations are small because the logic is strict. The plainest is the olive loaf with a tomato and oil, or with a sliver of cured ham and a soft cheese, the bread carrying the savoury weight. From there it leans toward the open tartine and the warm-weather filling: roasted vegetables, fresh cheese, herbs, the things a southern kitchen has to hand. It belongs with the broader tradition of French sandwiches built on a bread other than the baguette, gathered under Pain Garni & Non-Baguette Breads, and it is the entry where the loaf is the most thoroughly pre-seasoned in the set: the olives baked in, not laid on.