Heat oven to 425°F. Heat a large oven-safe skillet on medium-high. Rub steaks with 2 teaspoons olive oil, then season with ½ teaspoon each salt and pepper and cook until browned, 2 to 4 minutes per side. Transfer skillet to oven and roast steaks to desired doneness, 4 to 6 minutes for medium-rare. Transfer steaks to a cutting board and let rest for at least 5 minutes before slicing. Meanwhile, in a large bowl, whisk together vinegar, mustard, remaining 2 tablespoons oil, and ¼ teaspoon each salt and pepper; stir in scallion and horseradish. Add apple and cucumber and toss to combine. Fold in watercress and serve with sliced steak.