Preheat the grill to medium-high heat.

Season the flank steak with salt and pepper.

Grill the steak for about 4-5 minutes on each side for medium-rare, or until desired doneness.

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

In a large salad 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 small bowl, whisk together olive oil, red wine vinegar, Dijon mustard, minced garlic, salt, and pepper.

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