Place a 12-inch cast-iron skillet over medium-high heat 2-3 minutes or until hot. Coat steak with 2 Tbsp. oil, and season both sides of steak with 1 1/2 tsp. salt and 1 tsp. pepper. Cook steak in hot skillet 3-4 minutes on each side or until a meat thermometer inserted into thickest portion registers 130° (medium-rare). Transfer steak to a cutting board, and cover loosely with aluminum foil; let stand 5 minutes. Meanwhile, make chimichurri by stirring together onion, garlic, remaining 1 tsp. salt, and remaining 1 tsp. pepper in a small bowl. Stir in vinegar; let stand 5 minutes. Stir in parsley. Add remaining 3/4 cup oil in a slow, steady stream, whisking constantly until smooth. Cut steak diagonally across grain into ¼-inch-thick slices, and place on a serving platter. Pour chimichurri over sliced steak, and serve immediately.