Smoked salmon! My absolute favorite food! Just seeing those images makes my mouth water. (I know that what's shown is actually raw salmon filet, not smoked salmon, but that's OK since they both taste great.)
Here's a re-post of the smoked salmon sandwich of Cafe Luxembourg:

There was also this little shop along Rue Cler that, from what I saw in its window display, sold nothing but smoked salmon. I was amazed to find a store specializing in that particular food. Unfortunately, I don't remember its name...