For the Chimichurri Sauce, place onion, parsley, oil, vinegar, 1 teaspoon of the garlic salt and red pepper flakes in food processor or blender; cover. Process just until coarsely chopped. Set aside.
Sprinkle steak with remaining 1 1/4 teaspoons garlic salt and seasoned pepper.
Grill over medium-high heat 8 minutes per side or until desired doneness. Serve sliced steak with Chimichurri Sauce.