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.

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

In a small bowl, whisk together olive oil, red wine 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 with vinaigrette and toss gently to combine.