Preheat grill to 500 degrees.
Drizzle tuna fillets with 2 tablespoons of olive oil and season with salt and pepper.
Grill on each side for 2-3 minutes until cooked to medium rare. Set aside to rest for 5 minutes.
In a large mixing bowl, toss together fennel, apple, celery, walnuts and gorgonzola.
Drizzle with remaining olive oil and cider vinegar and season with salt & pepper.
Transfer the salad to a plate and top with grilled tuna