Brush rib-eyes with mustard on both sides. In a shallow bowl, or on a plate with a deep center, combine crushed peppercorns, ground pepper, if using, and salt, to taste. Pat pepper mixture onto steaks and crust thoroughly on all sides.
Prepare your grill by lightly spraying the grate with nonstick cooking spray and then preheat the grill. When the grill is hot, place the rib-eyes on the grate and cook until desired doneness, about 3 to 4 minutes on each side for rare. Add 1 to 2 minutes for medium-rare and so on. The best way to test steaks for doneness is to poke it with your index finger; rare will be soft, medium-rare will be slightly less soft, and so forth. Never flip a steak more than once; let it cook for half the total cook time before flipping. Remove from grill and let rest for 5 minutes in a warm place before serving.