In a small bowl, whisk together balsamic vinegar, olive oil, minced garlic, salt, and black pepper. Place the flank steak in a shallow dish or resealable bag and pour the marinade over it. Marinate for at least 30 minutes, or up to 4 hours in the refrigerator for more flavor.

Preheat the grill to medium-high heat. Remove the steak from the marinade and discard the marinade. Grill the steak for about 5-7 minutes on each side for medium-rare, or until it reaches your desired doneness. Let the steak rest for 5 minutes before slicing it thinly against the grain.

While the steak is resting, prepare the salad. In a large bowl, combine the mixed greens, cherry tomatoes, corn, red onion, and walnuts if using. Toss gently to mix.

Divide the salad mixture among serving plates. Top each salad with the sliced steak and sprinkle with crumbled Gorgonzola cheese.

Serve immediately, drizzling with additional balsamic vinegar or olive oil if desired.