Rub a generous amount of salt and pepper into both sides of the steaks. Rub the onion powder into the steaks, dividing it evenly. Put the steaks into a baking dish or container that holds them comfortably. Sprinkle the vinegar over the steaks and brush them with the olive oil. Marinate the steaks at room temperature for up to 30 minutes or refrigerate for up to 2 days.
Heat a gas grill to medium-high or heat a large grill pan over medium-high heat. Grill the steaks, turning only once, to desired doneness. Remove from the grill and let rest 5 minutes. If desired, slice the steaks thinly against the grain before serving. Drizzle some of the chimichurri over the steaks and pass the rest separately.
In a food processor, pulse the parsley and garlic until finely chopped. Scrape into a bowl and stir in the olive oil and vinegar. Season with salt and pepper, to taste. Add the red pepper flakes, if using, for a spicy chimichurri.
Yield: about 1 cup