Preheat the grill to medium-high heat. Season the flank steak with salt and black pepper.

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

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

In a large salad bowl, combine mixed greens, cherry tomatoes, cucumber, and avocado.

Slice the grilled steak against the grain and add it 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.