The bun is a soft rectangular roll. Sometimes the bun is heated, sometimes it is not. To toast the bun, you open it up without breaking it, butter both sides on the inside, and then griddle it buttered-side down in a hot frying pan.
Sometimes special versions are made with chopped chicken (“guédille au poulet”), chopped egg (“guédille aux oeufs”), chopped ham (“guédille au jambon”), or coleslaw.
With lobster (“guédille au homard”), it becomes what is called in New England a “lobster roll”: lobster meat mixed with chopped lettuce and mayonnaise on a warmed hot dog bun.
A vegetarian version of a Guédille might be coleslaw and tomatoes.
Some people are making really fancy ones now, such as with smoked salmon, rocket and goat’s cheese (“guédille au saumon fumé et fromage de chèvre”.) They even swap out the hot dog bun, and use a ciabatta roll. It could be questioned whether such a sandwich still qualifies as a Guédille.
“Guédille” means “snot” in French. While the standard French word is “morve”, in Anjou and in Poitou France, it is either guédille, gadille or guédille. Somehow the word landed in the New World. While in Québec the word definitely still retains its original meaning, it’s also been applied to this sandwich.
Some people say “goudille” instead, because it sounds more polite.