Preheat the grill to medium-high heat.

Season the flank steak with salt and pepper.

Grill the steak for about 6-7 minutes per side for medium-rare, or until desired doneness.

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

In a small bowl, whisk together olive oil, balsamic vinegar, Dijon mustard, minced garlic, salt, and pepper.

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

Slice the grilled steak against the grain and add it to the salad.

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