Prep: 20 min. + marinating Grill: 15 min. + standing
1 large red onion, sliced
1 cup minced fresh cilantro
1/4 cup white wine vinegar
1/4 cup Key lime juice
3 tablespoons extra virgin olive oil, divided
6 Key limes, halved
1 beef flank steak (1 pound)
1 teaspoon kosher salt
1/8 teaspoon pepper
In a small bowl, combine onion, cilantro, vinegar, lime juice and 2 tablespoons oil until blended. Pour 1 cup marinade into a large bowl or shallow dish. Add lime halves. Rub steak with remaining oil; sprinkle with salt and pepper. Add to bowl; turn to coat. Refrigerate 8 hours or overnight. Cover and refrigerate remaining marinade.
Drain steak, discarding marinade and limes in bowl. Place reserved marinade in a food processor; process until chopped chopped.
Grill steak, covered, over medium heat or broil 4 in. from heat until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°), 6-8 minutes per side. Baste occasionally with reserved marinade. Let stand 10 minutes before thinly slicing steak across the grain.