Season the steak surfaces with sea salt and black pepper.
Heat one tablespoon of olive oil in a cast-iron skillet over medium-high heat until the oil reaches its smoke point.
Sear the steak for 4 minutes per side for medium-rare internal temperature.
Transfer the steak to a resting rack and allow it to rest for 5 to 10 minutes to redistribute juices.
Prepare the dressing by whisking balsamic vinegar, extra virgin olive oil, and Dijon mustard until a stable emulsion is formed.
Arrange mixed greens, tomatoes, and red onions in two large serving bowls.
Slice the rested steak against the muscle fibers into 0.25-inch thick strips.
Distribute the steak slices over the salad beds.
Garnish with blue cheese crumbles and drizzle with the balsamic vinaigrette immediately before service.