Scale is the whole point of this one: a sausage broad and coarse enough that it leads outright and the bread falls back to support. The Lyonnais curing tradition runs to large-format pork sausages: the rosette, named for the shape of the casing it is cured in, and the Jésus, a thicker, knobbier cousin, both coarsely ground, generously fatted, and frequently studded with whole pistachios or a measure of black pepper. Some are dried like a standard saucisson sec; the cooked styles are poached and sliced warm. The build is a split baguette, often a thin spread of beurre demi-sel, and the saucisson sliced thick and laid in shingles along the crumb.
The logic follows from how substantial these sausages are. A rosette is wide enough that a single coin nearly covers the bread, marbled enough that the fat coats the palate, and coarse enough that you taste the grind rather than a smooth paste. That heft means the sandwich is sliced thicker than a standard saucisson sec build and stacked more sparingly: a few generous coins do the work of a dozen thin ones. The pistachios, where present, give green, faintly sweet pockets against the cured pork that are the signature of the Lyonnais style and the reason to seek this version specifically. Butter bridges the fat of the sausage to the crust and rounds the salt; the cooked styles, sliced while still warm, read softer and rounder than the dried ones, which keep more bite.
The bread needs a firm crust and an open crumb, because a wide slab of dense sausage will tear a weak loaf. There is no waiting and no assembly beyond slicing and stacking. A few cornichons cut the richness; a glass of something cool from the same region is a separate and welcome matter.
Variations move between the dried and cooked Lyonnais styles and the inclusions. A pistachio-studded rosette is the signature; a plain peppered version reads leaner and sharper; a warm cooked-and-sliced sausage turns it softer and more savory. It belongs with the cured-meat builds the catalog groups under Sandwich Saucisson & Charcuterie, and its specific contribution is scale: a coarse, fat-marbled, often pistachio-flecked sausage substantial enough that the sandwich is organized around the meat rather than the bread.