Although the marinade helps tenderize this large cut of meat, you should also make sure to slice it very thinly against the grain on a slight bias. We found that a serrated knife works best.
• 1 cup cola
• 1/4 cup red-wine vinegar
• 1/2 small white onion, grated
• 1 1/2 tablespoons garlic, chopped (from 3 garlic cloves)
• Coarse salt and freshly ground pepper
• 1/4 cup safflower oil, plus more for grill
• 1 London broil (5 to 6 pounds; 2 1/2 inches thick)
Combine cola, vinegar, onion, garlic, 1 teaspoon pepper, and the oil in a bowl. Place steak in a large resealable plastic bag, and pour marinade over steak. Seal bag, pressing out excess air. Refrigerate at least 8 hours.
Let steak (in bag) stand at room temperature 1 hour. Remove steak from marinade, and discard marinade. Brush off any remaining onion and garlic from steak to prevent burning, and season with 1 tablespoon salt and 1 teaspoon pepper.
Meanwhile, preheat grill to medium-high, and brush grates with oil. Grill steak until internal temperature reaches 115 degrees for medium-rare (residual heat will continue to cook meat), 10 to 11 minutes per side. Transfer steak to a cutting board, and let rest, tented with foil, at least 15 minutes and up to 1 hour. Very thinly slice steak against the grain, and serve warm or at room temperature.