Preheat the grill to medium-high heat.

Season the flank steak with salt and pepper.

Grill the steak for about 5 minutes on each side for medium-rare.

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

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

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

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

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