Add the onions, garlic, and 2 tablespoons of olive oil into a pan. Cook on medium heat until the onions are golden brown.
Add the onions and all of the other chimichurri sauce ingredients into a food processor. Puree until smooth. Season well with salt and pepper
marinade the steaks with the chimichurri sauce and let sit for a couple of hours
Season the steaks well with the blackening spice and salt. Grill on the barbecue or grill for 3 minutes per side. Aim for medium rare. There should be dark pink juices coming out of the top of the steak while it rests.
Let the steaks rest for a few minutes and then slice and serve with extra chimichurri sauce on top.