Whisk the orange juice, lime juice, garlic, cumin and 1 teaspoon salt, add the flank steak and refrigerate for 1 to 2 hours.
Remove the steak from the marinade, pat its surface dry and discard the marinade.
Heat the olive oil in a cast-iron skillet over medium-high heat and cook the onion with the remaining salt for 6 minutes until soft and browned.
Transfer the onion to a plate 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 with the onion.
Butter the outsides and press the sandwiches in the skillet for 1 to 2 minutes per side until crisp and compact.