The Sandwich Rosette de Lyon is named for one specific sausage, and the sandwich is little more than a frame for slicing it well. The rosette de Lyon is a large-format dry-cured pork sausage from Lyon, stuffed into the wide end of the pig's hind casing and cured slowly until it is firm all the way through. The casing is the point of the name: the rosette is the broadest section of the gut, so the sausage emerges as a thick cylinder that slices into coins nearly as wide as the bread. The grind is coarse, the lean is generously veined with white fat, and the cure is long enough that a slice is concentrated rather than soft. The build is a split baguette, often a thin spread of beurre demi-sel, and the rosette laid in broad shingled coins with little else, so the cured pork is the whole sandwich.
The logic follows from the diameter. Because a single coin nearly covers the crumb, the sandwich is stacked sparingly: a few wide slices do the work of a dozen narrow ones, and cutting the rosette thin wastes the coarse grind, the slice going waxy and losing its chew. The fat is the carrier, marbled through every coin so it coats the palate and bridges the salt of the cured pork to the wheat of the crust, which is why butter is welcome but not strictly required, and why a well-veined rosette reads round where a lean one reads sharp and flat. The bread does the structural work, so it needs a firm crust and an open crumb that pulls cleanly rather than tearing under a broad, dense slab. There is no warm component and no waiting: this is sliced, stacked, and eaten standing, with a few cornichons to cut the fat.
Variations move within the Lyonnais curing shelf rather than off it. The everyday everywhere-version of the format is treated separately as the Sandwich Rosette, and the broader Lyonnais large-sausage tradition the rosette sits inside, including the thicker knobbier Jésus and the pistachio-studded styles, is its own reading as the Sandwich au Saucisson de Lyon. Each holds the coarse, fat-veined dried sausage as the fixed point and changes only the format or the inclusions. It belongs with the cured-meat sandwiches the catalog groups under Sandwich Saucisson & Charcuterie. Its specific contribution is the rosette in its own casing: a wide, coarse-ground, fat-marbled Lyonnais dry sausage broad enough that the sandwich is organized around the slice rather than the bread.