How to Make It
Stir together salt, black pepper, and 2 teaspoons sugar in a small bowl; rub mixture on steak. Let stand for 10 minutes. Whisk lime juice, fish sauce, crushed red pepper, shallot, and remaining 2 teaspoons sugar in a large bowl.
Heat oil in a large cast-iron skillet over medium-high. Cook steak, in batches if necessary, flipping once, until charred and medium-rare, about 2 minutes per side. Transfer to a cutting board; let rest for 10 minutes before slicing.
Add romaine, tomatoes, and ¼ cup cilantro to bowl with lime dressing and toss to coat. Serve steak over salad, topped with mint and remaining cilantro.