The boudin sandwich is defined by a sausage that is mostly rice. Cajun boudin is a soft link of pork, pork liver, cooked rice, onion, and seasoning, and the rice is not a filler in the sandwich sense but the body of the sausage itself, which is what makes putting it on bread an unusual move. The defining decision is whether the link stays in its casing or comes out of it, because that choice changes the sandwich from a sausage sandwich into something closer to a seasoned rice dressing on bread.
The craft is in handling a filling that has almost no structure. Boudin is poached or steamed soft, and the casing holds a loose, moist mix that does not slice or stack like a firm sausage. The simplest build lays the whole link on plain white bread or a soft bun, and the bread is chosen to disappear: a quiet, spongy carrier that soaks the rendered fat and gives the hands a grip while the seasoned pork and rice do the talking. The more common move squeezes the boudin out of its casing onto the bread, so the sandwich becomes a warm, well-spiced rice-and-pork mixture that the soft loaf has to contain rather than support. Either way the build stays minimal, because the link is already a complete, seasoned thing and the structural job of the bread is only to make it portable. Pickle, hot sauce, or Creole mustard is the usual accent, a sharp acidic counter to a rich, soft, liver-deepened filling that would otherwise read as one heavy note.
The variations stay close to Acadiana and to the link itself. The boudin ball breads and fries the same mix into a crisp sphere that is a different sandwich logic entirely; the smoked-boudin build firms the link over wood; the crawfish or seafood boudin swaps the protein while keeping the rice fixed. These belong to the dense long tail of regional American specialties tied so tightly to one corner of the country that they barely exist a state away, and each deserves its own article rather than being crowded in here.