Inspired loosely by the flavors of shaking beef—the tangy, slightly sweet Vietnamese stir-fry—this London broil gets lots of flavor from a mixture of soy, sesame, ginger and garlic that’s rubbed into little slits cut into the meat before cooking. Top round, which is typically a tough cut of meat, is best served rare or medium-rare and sliced super-thin. The vibrant onion and cucumber salad is the perfect accompaniment to the sliced beef. Unlike many recipes for London broil, this version doesn't require marinating the beef, but you can certainly coat it in the marinade and refrigerate it for few hours before broiling it.
Position an oven rack or broiler pan about 6 inches from the heat source. Preheat the broiler to high.
Place the steak on a large baking sheet or on the broiler pan. Using a paring knife, cut 1/8-inch-deep slits in a crosshatch pattern on both sides of the steak. Season the meat all over with salt and pepper.
Whisk the soy sauce with the sesame oil, ginger, garlic and sugar together in a small bowl. Spoon half the soy mixture on one side of the steak and rub it into the slits. Flip the steak and rub the remaining soy mixture into the slits on the opposite side.
Broil the steak until nicely browned on one side, about 5 minutes. Flip and broil until browned on the other side and an instant-read thermometer inserted in the thickest part of the steak registers 120 degrees F for rare or 125 degrees F for medium-rare, about 5 minutes more. Transfer the steak to a cutting board and let rest for 5 to 10 minutes.
Meanwhile, thinly slice the cucumbers and transfer them to a large bowl. Very thinly slice the onion crosswise, then separate the slices into rings. Add the onion, vinegar, oil and a generous pinch each of salt and pepper to the bowl; toss well. Fold in the lettuce and season to taste with salt and pepper. Thinly slice the steak against the grain and serve alongside the salad.