Preheat the grill to medium-high heat.

Season the flank steak with salt, pepper, and garlic.

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 large mixing bowl, combine mixed greens, cherry tomatoes, cucumber, avocado, and red onion.

Slice the grilled steak against the grain into thin strips.

Add the steak to the salad bowl.

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.

Serve the steak salad immediately, garnished with additional avocado if desired.