The Sandwich Bayonne-Ossau-Iraty is built on a single regional pairing, and the pairing is the whole sandwich. Jambon de Bayonne is the air-dried ham of the southwest, salt-cured and matured to a deep, slightly sweet savor and sliced thin enough to drape; Ossau-Iraty is the firm sheep's-milk cheese of the same country, dense, nutty, and faintly grassy. On a crusted split loaf, the thin ham and a layer of the sheep cheese are set against each other, usually with little else, so the cure and the dairy are what you taste.
The logic follows from the two components answering each other. The ham is lean and intensely savory from its long air-dry, the cheese rich and firm with a nutty depth, so each supplies what the other lacks: the cheese's fat rounds the ham's salt, and the ham's savor cuts the cheese's density. The pairing is self-balancing, which is why the build stays close to bare and a butter or a strong condiment would only get in the way. The constraint is the slice and the bread. The ham has to be cut thin enough to fold, because at full thickness the cure turns relentless and the fat goes waxy; the loaf needs a real crust because firm cheese and draped ham give the sandwich no structure of its own. It eats cool and clean, firm rather than melting, a sandwich that rewards restraint and punishes the urge to build.
Variations stay inside the southwestern larder rather than wandering off it. A version with a smear of black cherry preserve, the regional partner to the sheep cheese, adds a sweet-tart note against the salt; one leaning on a more aged Ossau-Iraty pushes the nutty depth further; the plainest is ham, cheese, and bread alone. Each holds the Bayonne-and-Ossau-Iraty pairing as the fixed point and changes only what frames it. The Sandwich Bayonne-Ossau-Iraty belongs with the cheese sandwiches the catalog groups under Baguette Fromage, the tradition that gives each regional cheese its own treatment on bread. Its specific contribution is a self-balancing pair of one cured ham and one sheep cheese from the same country, where the sandwich's job is to set them against each other and add nothing.