Heat broiler, with rack 6 inches from heat. Pat steak dry with paper towels, season with salt and pepper, and rub with oil. Place on a foil-lined rimmed baking sheet. Broil 6 minutes per side for medium-rare. Transfer to a cutting board; tent loosely with foil and let rest 10 minutes.
Meanwhile, in a large bowl, whisk buttermilk, mayonnaise, and vinegar. Season. Thinly slice steak against the grain. In a bowl, toss greens with steak, pears, pecans, and dressing.