Drizzle filet with olive oil and season with salt and black pepper. Sear in a cast-iron pan over medium heat 6 minutes per side. Remove and let rest 7 minutes before slicing into bite-size pieces. Keep warm.
Toss onion with a little olive oil and sauté in same pan until lightly browned around edges.
To serve, place a big scoop of steak into a hoagie roll. Top with onion and torn cheese. Serve.