Preheat the grill to high heat. Season the steak with salt and pepper.
Grill the steak for about 3-5 minutes on each side, or until it reaches your desired level of doneness. Remove from grill and let it rest for 5 minutes before slicing thinly against the grain.
In a large salad bowl, combine mixed greens, carrot, red bell pepper, cucumber, and red onion.
In a small bowl, whisk together olive oil, balsamic vinegar, and Dijon mustard to make the dressing. Season with salt and pepper to taste.
Add the sliced steak to the salad. Drizzle the dressing over and toss gently to combine. Top with crumbled feta cheese if desired and serve immediately.