The Pain aux Noix Garni is built around a bread that has already chosen its partner. Pain aux noix is a wheat or part-rye loaf with whole and broken walnuts worked through the dough, so the crumb is studded with pockets of bitter, tannic, slightly oily nut. That tannin is what a baguette cannot give a sandwich, and it points straight at cheese. Walnut and blue are an old understanding: lay a wedge of Roquefort or Fourme d'Ambert against a slice of this bread and the salt of the cheese, the bitterness of the nut, and the faint sour of the crumb settle into each other with no help required.
The filling logic is narrow on purpose. The walnut bread is loud, so it works with fillings that either match its intensity or lean on its sweetness elsewhere: blue cheese, a ripe goat cheese with honey, a thin slice of jambon de pays, a fresh white cheese with grapes or pear. It resists a mild ham or a wet salad, which the nuts would overpower. The crumb is denser and heavier than a baguette's, weighed down by the nut pieces, so it carries a rich filling without going slack, and it tears in a clean dense way rather than flaking. A few walnut halves are often added on top of the cheese, doubling the bread's own seasoning, which is the surest sign that the loaf and the filling are meant to be read as one thing rather than bread plus topping.
The variations stay close to the cheese board. The plainest is walnut bread, butter, and a single strong cheese, eaten the way you would eat the end of a meal rather than a lunch on the move. From there it moves toward the open tartine, the slice toasted to bring the nut oil forward and topped with warm goat cheese and honey, and toward the autumn pairings of fig, grape, and pear that echo the bread's natural sweetness. It sits within the broader family of French sandwiches that begin somewhere other than a baguette, collected under Pain Garni & Non-Baguette Breads, and it is the entry where the bread has effectively pre-selected the cheese before the sandwich is even assembled.