Preheat the grill to medium-high heat.

Season the flank steak with salt and pepper.

Grill the steak for about 5-7 minutes on each side for medium-rare, or until desired doneness.

Remove from the grill and let it rest for 5 minutes before slicing.

In a large salad bowl, combine mixed greens, cherry tomatoes, cucumber, red onion, and feta cheese.

Slice the grilled steak against the grain into thin strips and add to the salad.

In a small bowl, whisk together olive oil, red wine vinegar, Dijon mustard, minced garlic, salt, and pepper to make the vinaigrette.

Drizzle the vinaigrette over the salad and toss gently to combine.