Ficelles are thin baguettes, named after the French word for string. If you can't find one, use a baguette.
Heat oil in a large skillet over medium heat. Add franks, and cook, turning occasionally, until golden brown and heated through, 6 to 8 minutes.
Spread cut sides of bread with relish. Place franks, end to end, on top of relish. Cut sandwich into 6 equal portions, and serve with yellow mustard.