For sauce, process parsley, onion and garlic in food processor or blender 30 seconds or until evenly chopped. Add Hellmann's® or Best Foods® Real Mayonnaise and vinegar. Continue processing, scraping down sides, 30 seconds or until combined. While processing, add olive oil and red pepper flakes and process until blended. Season, if desired, with salt and ground black pepper; set aside.
Sprinkle steak with salt and pepper. Grill or broil steak, turning once, until desired doneness. Serve sliced steak with Creamy Chimichurri Sauce.