Heat broiler. Season the steak with ½ teaspoon each salt and pepper and place on a rimmed broilerproof baking sheet.
Broil to desired doneness, 3 to 4 minutes per side for medium-rare. Let rest at least 5 minutes before slicing.
Meanwhile, in a large bowl, whisk together the mustard, vinegar, lemon juice, oil, cilantro, and ¼ teaspoon each salt and pepper.
Add the arugula, bean sprouts, broccoli slaw, scallions, red onion, radishes, and green beans and toss well. Top with the beef before serving.