These Mexican-style grilled steak sandwiches are a great weeknight or cookout party option. Flat iron is a super-flavorful cut from the shoulder; it’s quite lean, so cook it no further than medium-rare to ensure that it’s tender and juicy.
Prepare a gas or charcoal grill for high-heat cooking. Sprinkle steak with salt and pepper and then grill, turning once or twice until browned and cooked to medium-rare, about 8 minutes. Let rest 5 minutes, then slice very thinly across the grain. Pile steak on bottoms of rolls and top with lettuce, carrot, avocado and cilantro. In a small bowl, stir together yogurt, lime juice and chopped jalapeños, and drizzle the mixture into the sandwiches. Add more pickled jalapeño to the sandwiches if desired.