Heat a grill to medium-high.
Meanwhile, sprinkle the steak with salt and pepper. Rub the tomatoes and onion slices with the oil and sprinkle with salt and pepper.
Grill the steak, turning once, until medium-well (12 to 15 minutes). Transfer to a cutting board and let rest for 5 minutes. Grill the tomatoes and onions next to the steak, turning once, until charred and softened, about 5 minutes for the tomatoes and 10 minutes for the onions.
Transfer the tomatoes to a cutting board. When cool enough to handle, cut into wedges and transfer to a bowl, along with the Worcestershire sauce. Toss until well mixed.
Cut the steak into thin slices against the grain and transfer to a serving platter, along with the onions. Spoon the tomatoes with their juices all over and sprinkle the cheese and parsley on top.