Freeze the steak for 30 minutes, then slice it paper thin against the grain. Half-frozen beef cuts cleanly; soft raw beef does not.
Stir the grated pear, soy sauce, brown sugar, sesame oil, garlic, ginger, and black pepper together. Toss the beef in the marinade and let it sit for 30 minutes at room temperature, or up to overnight in the fridge.
Heat the neutral oil in a wok or large skillet over high heat until it just starts to smoke.
Cook the beef in two batches. Spread it in a single layer, leave it for 60 seconds to char, then toss for another minute until cooked through. Crowding the pan turns the marinade into a stew, so keep the batches honest.
Return the first batch to the pan, add the scallions, and toss everything off the heat for 30 seconds.
Sprinkle with sesame seeds and serve hot, with the lettuce leaves on the side for wrapping at the table.