In DC, visit The Breadline at 1751 Pennsylvania Ave, NW. The menu describes their sandwich as "Tuna Salad:
On olive bread, with the flavors of Morocco."

Don't ask for any substitutions, unless you want to feel the wrath of "crusty" Mark Furstenberg, the guy who introduced high-quality traditional breads to Washington, DC when he started Marvelous Market all those years ago.