The hot sausage po' boy is built on a sausage that is fried as a flat patty, not served in its link. New Orleans hot sausage is a fresh, coarsely ground, heavily peppered Creole pork (sometimes pork and beef) sausage, and for this sandwich the meat is taken out of the casing, pressed into a patty, and griddled or pan-fried until the edges are dark and crisp. Frying it flat is the defining decision: it maximizes the crusted surface, renders the spiced fat to the bottom of the pan, and gives the loaf a single dense, juicy slab rather than a row of links rolling out the side.
The craft is in the fry and in what the rendered fat is allowed to do. The sausage is loose and fatty, so the patty has to be cooked hot enough to set a seared crust while the pepper-laced fat renders rather than steaming the meat gray. Some kitchens spoon a little of that hot, red, rendered grease back onto the loaf, treating it as part of the dress rather than waste, which is why the build reads spicier than the sausage alone. The thin-crusted, glass-crackly New Orleans loaf is chosen because it shatters cleanly under the bite and its airy crumb soaks the rendered fat without going to paste for the length of the sandwich. Dressed means shredded lettuce, tomato, pickle, and mayonnaise as the cool, acidic counter to a fatty, peppery patty, and yellow or Creole mustard is a common standing addition. A po' boy shop builds these straight off the flat-top so the crust is still firm and the fat still warm in the crumb.
The variations track the heat and the build. A linked-sausage version keeps the casing and changes the texture entirely; a cheese addition mellows the pepper; an egg turns it into a breakfast po' boy. Those readings, along with the roast-beef debris, fried-seafood, and cochon de lait po' boys in the same family, are their own sandwiches and deserve their own articles rather than being crowded in here.