In a medium bowl combine the parsley, cilantro, onions, garlic, olive oil, vinegar, oregano, salt, pepper, and chili flakes. Stir and set aside.
Grilled Bavette
Remove the bavette steak from the refrigerator 30 minutes before you are ready to cook it. Preheat the grill to 425 degrees F (220 degrees C).
Spray the bavette with the duck fat spray. Season liberally with salt and pepper.
Grill the steak until it reaches your desired doneness, 3 to 4 minutes per side for medium-rare.
Remove the steak to a cutting board and tent it with aluminum foil. Let rest for 5 to 10 minutes before slicing thinly against the grain. Serve the steak topped with the chimichurri sauce.