Preheat the grill to medium-high heat.

Season the flank steak with salt and pepper on both sides.

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

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

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

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

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

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

Serve immediately.