Season the hanger steak with 3/4 teaspoon salt and the black pepper.
Heat half the oil in a skillet over medium heat and cook the red onion with the remaining salt for 10 to 12 minutes until soft and browned.
Stir the balsamic vinegar into the onion for 30 seconds, then transfer the mixture to a bowl.
Raise the heat to medium-high, add the remaining oil and sear the steak for 4 to 5 minutes per side until it reaches 145°F.
Rest the steak for at least 3 minutes, slice it thinly across the grain and divide it among the rolls.
Top each sandwich with browned onion and 1 ounce blue cheese, then broil for 1 minute until the cheese softens but does not fully liquefy.