Preheat an outdoor grill to medium-high heat.
Mix together the olive oil, garlic powder, onion powder, paprika, salt and pepper. Set aside.
Trim excess leaves off the cauliflower head. I like to leave some on as they crisp up and char really nicely on the grill. Stand the cauliflower stem side up and make vertical slices to cut the cauliflower into steaks about 1-1.5 inches thick. I keep the extra florets and grill them in a veggie basket so they don't go to waste. Or you can reuse the extra florets in another recipe.
Baste each of the cauliflower steaks with the oil and spices. Place on the grill oil side down and baste the other side. Grill for 6-7 minutes per side for a crunchier steak, or grill 7-9 minutes for a softer chew and more char.
While the steaks are grilling, prepare the chimichurri. Mince the parsley and mix with the olive oil, garlic, red wine vinegar, salt, pepper, oregano, and red pepper flakes if using.
Remove the steaks from the grill and top with the chimichurri. Serve warm with extra chimichurri on the side.