Heat a grill or grill pan over medium-high heat. Season the steak with salt and pepper. Grill the steak for 4-5 minutes per side, or until desired doneness. Let rest for 5 minutes before slicing.
In a small bowl, whisk together the olive oil, red wine vinegar, Dijon mustard, salt and pepper. Set aside.
In a large bowl, combine the mixed greens with the dressing and toss to coat.
Divide the salad among four plates. Top each plate with sliced steak.