Kosher or coarse salt and freshly ground black pepperto taste
Using a sharp knife stab the steaks to create little incisions all over the meat, or use a bladed meat tenderizer. Combine the onion, thyme, mustard, Worcestershire sauce, olive oil, and salt and pepper in a large sturdy zipper top bag or large container. Place the steak in the marinade and turn to coat well. Cover and refrigerate for at least 4 hours, and up to 12.
Let the steak come to room temperature for at least 30 minutes before cooking. Preheat the grill or heat a grill pan over high heat. Grill the steak, covered, until the meat is well browned on the outside but medium rare on the inside, 4 to 5 minutes per side. Let the meat rest for 5 minutes before thinly slicing across the grain.