1. Preheat broiler.
2. Sprinkle both sides of flank steak evenly with garlic, salt, and 1/4 teaspoon black pepper. Place steak on a broiler pan coated with cooking spray; broil steak 5 minutes on each side or until desired degree of doneness. Place steak on a cutting board; cover and let stand 5 minutes. Uncover; cut steak diagonally across grain into thin slices.
3. Combine pepperoncini pickling liquid, olive oil, and mustard in a medium bowl, stirring well with a whisk. Add chopped pepperoncini pepper, parsley, cheese, and cucumber to oil mixture in bowl; toss well to combine. Serve steak with relish.
Pepperoncini peppers are yellow, wrinkled, and slightly spicy; we use both the chopped pickled pepper and pickling liquid to flavor the crunchy relish.