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, or until desired doneness.

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

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

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

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

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