Season the steak with salt and press the cracked pepper firmly onto both broad sides.
Heat the oil in a heavy skillet over medium-high heat and sear the steak for 4 to 6 minutes per side until it reaches 145°F.
Transfer the steak to a board and rest it for at least 3 minutes.
Mix the softened butter and Dijon, spread it over the baguette sections and toast them cut-side down in the skillet for 2 minutes.
Slice the rested steak very thinly across the grain and divide it among the baguettes.
Top each with 1/4 cup romaine and close the sandwiches without pressing out the steak juices.