I was trying to go to a bistro called Le Comptoir du Relais; it was completely full (it almost never occurs to me to make reservations, so this actually happens a lot). They do, however, have a small take-out window with sandwiches and pastries. I figured this was a pretty good opportunity to try a ham and butter sandwich, which is supposedly the second most popular sandwich in France.
(The first? Burgers. Everyone loves burgers.)