Preheat oven to 350 degrees F (175 degrees C). Grease a loaf pan.
Place pork sausage, bread crumbs, onion, egg, and shallot in a large bowl. Season with salt and pepper. Pour in buttermilk; mix thoroughly. Pat mixture gently into the prepared loaf pan; top with ketchup.
Bake in the preheated oven until edges are bubbling, about 45 minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (71 degrees C). Cool for 5 minutes before slicing.