The Panini Jambon-Fromage is defined by the press as much as by the filling. The bread is the soft, slightly sweet Italian-style roll the French call pain à panini, split, layered with jambon blanc and a meltable cheese (usually emmental, sometimes a mild raclette or mozzarella), then closed and clamped onto a ridged hot plate until the outside carries dark griddle lines and the inside has gone soft and pliable. What goes in is the most ordinary pairing in the French repertoire, ham and cheese. What makes it a panini is that the whole thing is then flattened and heated until the cheese fuses the two halves into one.
The press does specific work, and the filling is chosen to survive it. Jambon blanc holds its shape under heat without weeping much liquid, so the bread stays structurally sound rather than steaming itself soft from the inside. The cheese has to be a melter: it needs to run between the slices of ham and into the open crumb of the roll, then set just enough on cooling to act as the glue holding the closed sandwich shut. The bread itself is enriched and a little sweet, which is why it takes color so fast on the plate and why the finished panini reads as warm and mild rather than sharp. It comes off the press too hot to bite immediately, goes leathery if it sits, and is meant to be eaten standing within a minute or two of the plate opening. This is counter food: ordered, pressed while you wait, handed over in a paper sleeve.
Variations keep the press and swap what gets clamped inside it. The chicken version and the tuna version are the two most common and are treated in their own entries rather than crowded in here. Within the ham-and-cheese frame the usual moves are a few rounds of tomato, a smear of pesto, or a swap of emmental for mozzarella to push it closer to the Italian original. The Panini Jambon-Fromage belongs with the pressed and non-baguette breads the catalog groups under Pain Garni & Non-Baguette Breads, the shelf of French sandwiches that aren't built on a baguette. Its particular contribution is the hot plate: a sandwich where the defining technique is compression and heat, and the filling's only job is to melt cleanly and not fight the bread.