Preheat the grill to medium-high heat.

Season the flank steak with salt and pepper.

Grill the steak for about 5-7 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 salad bowl, combine mixed greens, cherry tomatoes, cucumber, red onion, and avocado.

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

In a small bowl, whisk together olive oil, red wine vinegar, Dijon mustard, minced garlic, salt, and pepper to make the dressing.

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