1. Place the onion slices into a medium bowl. Toss with the sumac to coat all the slices. Set aside.
2. Place the steak into a glass container or baking dish just large enough to hold it. Massage in the Worcestershire sauce and three tablespoons olive oil to coat both sides.
3. Heat one tablespoon olive oil in a large grill pan or skillet over medium-high heat. Season the steak with salt and pepper. Add onion to the pan; cook until tender, about four minutes. Push onions to the side; add the steak, searing three to six minutes per side or until medium rare. Thicker London broil will take much longer and the onions may need to be removed from the pan if they start to burn. Transfer steak to a cutting board; allow to rest for 10 minutes before slicing.
4. Meanwhile, prepare the dressing: In a medium bowl whisk the pomegranate syrup, vinegar, mustard, salt and pepper. Whisk in the remaining three tablespoons extra-virgin olive oil.
5. Arrange the greens and tomatoes on a platter. Drizzle with just enough dressing to coat lightly. Slice the steak across the grain; arrange on and in the salad. Toss the onions on top. Drizzle with additional dressing.