I'm Belgian and this recipe is very poplar there. You eat warm on a cold fall or winter day. The original recipe calls for lardons but we were not big fans of lardons so my mother used other types of smoked meat, such as thin slices of Grison meat.
For vegetarians, you can "replace" the meat with baby tomatoes. One of the latest changes to this recipe in my family is to add some Garlic and Fine Herbs Boursin (Trademark), a French creamy cheese spread. It is particularly good if you are vegetarian. —MarieGlobetrotter