Jambon beurre
A jambon-beurre , also known as a parisien, is a French ham sandwich made of a fresh baguette sliced open, spread with butter (salted or unsalted) and filled with slices of ham. Every day in France, over 3 million jambon-beurre sandwiches are sold, more than almost any other kind of sandwich, except for fast-food hamburgers.
Source: Wikipedia